FEA TURE

STRATIGRAPHIC ANALYSIS A N D RESERVOIR PREDICTION IN THE EOCENE YEGUA A N D

COOK MOUNTAIN FORMATIONS OF TEXAS A N D LOUISIANA

By Marc B. Edwards, Consulting Geologist

ABSTRACT Predtcting the locations and geometry of downdip this purpose, well log, seismic and biostratigraphic data

sandstone reservoirs in the Yegua trend of Texas and were integrated over a distance of about 350 miles along Louisiana is a key challenge to explorationists. In order to strike. The resulting correlation framework, based on formulate and test depositional and stratigraphic models for almost 4,500 well logs, extends from updip on the stable

.- shelf t o the downdip limit of well control on the slope. Ed. Note 7 'hs artrcle IS reprinted with permission from the GCSSEPM Well logs provide the highest stratigraphic resolution in Foundatrori Eleventh Annual Research Conference, December 2, 1990. this data set . O n the stable shelf, the entire Yegua-Cook

Figure 1. Index map showing in box the location of the study area in Middle Texas Gulf Coast, and Southeast Texas and Southwest Louisiana.

3 7 Houston Geolog~cal Society B u l l e t ~ n December 1990

Mountain sand-bearing interval is about 1,000-1,500 ft thick. Shale markers which show tens to 100s of miles of continuity along strike bracket progradational cycles 100- 300' thick. These log-defined cycles comprise the basic mapping units used in this study. Seismic reflectors in this zone are essentially parallel. Downdip of the shelf, compli- cations were introduced by growth faulting, erosion due to slope failure, mass wasting of sediments overlying failure- induced erosion surfaces, and complex patterns of basin fill seaward of the shelf margin. This complex zone thickens basinward to several thousand feet. Biostratigraphic data were used to determine general ages for downdip section, to indicate prominent discontinuities, and to show environ- mental trends.

Large-scale slope failures occurred several times during Yegua deposition, resulting in regional erosional uncon- formities. It is not known if their origin is related to eustacy. Slope deposits that onlap extensive erosional surfaces are also well represented, but their precise age relationship to updip systems is unclear.

Paleoenvironments were interpreted from maps of log- derived interval isopach, net sand, percent sand, blocky sand and log facies. Each mapping unit is bounded by regionally correlatable shale zones interpreted to have formed during high stands. Strata bounded by the markers represent systems tracts comprising upper delta plain, lower delta plain, mouth bar, distal mouth bar, and shelf. Inferred local shelf bypassing, and channel incision and basinward shifts in facies suggest small but significant falls in sea level during deposition. Each of these cycles resemble "depositional sequences" but in terms of their duration they are described as high frequency fourth order events. Small- scale stratigraphic complexity within the mapped units suggests frequent autocyclic shifts of depocenters.

Effective sand prediction downdip to the shelf edge and upper slope is accomplished using transgressive shale ("condensed section" or "flooding surface") markers as a stratigraphic framework, while recognizing the significant effects of eustatic sea-level changes. Although wide-spread erosional unconformities could be mapped on the slope, neither unconformities nor correlative conformities could

be identified sufficiently to enable detailed and regional mapping on the shelf.

INTRODUCTION The downdip Eocene Yegua trend of Texas and

Louisiana is a comparatively recent development in the context of oil and gas exploration in the northern Gulf Coast Basin. Well log and seismic data have documented enor- mous stratigraphic and structural complexity in a trend that was virtually undrilled a decade ago. Understanding this complexity is an important element in effective prediction of reservoir sandstone distribution for hydrocarbon explo- ration.

The stratigraphic framework used was developed from detailed correlation and mapping from approximately 4,500 well logs, biostratigraphic analysis of cuttings for first downhole occurrences and environment of 62 wells, and interpretation of over 1,000 miles of recently acquired seismic data. Other types of data such as velocity surveys, dipmeter logs and whole cores were also locally incorpo- rated.

GEOLOGIC SETTING The study area extends from the San Marcos Arch in

the middle Texas Gulf Coast through the Houston salt province in southeast Texas to southwest Louisiana (Figs. 1 and 2).

Subsurface studies across the trend have described distributary channel and barrier bar deposits in southwest Louisiana (Lautier, 1981), river-dominated (Fisher, 1969), or wave-dominated (Kaiser et al., 1980) delta systems in Texas, and small wave-dominated to lobate deltas in South Texas (Van Dalen, 1981). These and other studies mapped a basinward decrease in sand that defined the downdip limit of hydrocarbon exploration in the Yegua.

In 1982, drilling revealed significant Yegua sands con- siderably downdip from the accepted zero sand limit. Whitten and Berg (1987) interpreted cores from Jackson County as hurricane-generated turbidites deposited in channels. Scott (1987) described autocyclic shifts, fault control of sand deposition and eustatic control of sedimen- tation in the Yegua. Edwards and Tuttle (1989b) described

SAN MARCOS SABIN€

ARCH R I M R - DOMINA TED

GENERALIZED DOWDIP LIMIT OF 'UPDIP' SANDS

UPOlP EDGE OF SLUUP 0 I 0 1 0 I 0 4 0 5 0 UILCS - i SALT CiAPIRS IN HOUSTON

SALT DIAPIR CMBAYUEMT

Figure 2. Regional map showing major structural provinces, generalized Yegua paleogeography, major shelf-edge slumps, salt domes, and unstable shelf edge along which downdip sandstones occur. Dotted lines show locations of cross-section figures 5 , 8 , 11 and 12.

Houston Geologncal Soctety Bullet~n. December 1990 38

large-scalv erosional unconformities in the Yegua-Cook Mountain section based upon logs, seismic and biostrati-

the core paleogeographic unit in which distributary systems are relatively close and mouth bars are coalesced (Fig. 2). Liberty Dome is near the geographic center of the Liberty delta system, which extends from Fort Bend County to Jasper County.

The flanks of the depocenter are characterized by comparatively isolated distributary channel networks and mouth bars as well as lower rates of progradation and sedimentation. In addition, map trends suggest more wave reworking and strike transport. These areas are termed the Wharton flank to the southwest of the depocenter, and the Beauregard flank to the east (Fig. 2).

Structurally, the study area includes three domains: 1, the Houston Salt Diapir Province; 2, the Wharton unstable shelf-edge; and 3, the Beauregard unstable shelf-edge. The

graphic d'ita. These features were interpreted as a result of large-scale catastrophic slope and shelf-edge failure.

In Louisiana, Lock and Voorhees (1988) mapped thin sand units in the upper part of the Yegua (Cockfield) and inferred a n open shelf environment with deposition by storms. In addition they described an apparent truncation at the top of the Cockfield section, which they interpreted to represent a downlap surface. An exploration overview of the downdip Yegua was provided by Ewing and Fergeson (1989).

The present work on the Yegua-Cook Mountain has focused on the central delta systems and flanking areas. In this paper the term "Liberty delta system" is proposed for

CYCLES

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REPORT m ---a

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LOG CORREUTION

MARKERS

EROSION SURFACES

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YEGUA

COOK

Cloborotolia spinuloso

Cloborotolio bullbrooki

Clowlino quoyobolensis

MOUNTAIN &bulinoldes beckmoni

Globwotolio broedennoni

Figure 3. Stratigraphic framework, nomenclature, principal biostratigraphic zones, eustatic curves and sequence boundaries with age, and depositional cycles, occurrences of major shelf-edge slumps and incised channel systems. All ages and Exxon eustatic curve from Haq e t al. (1988).

Houston Salt Diapir Province to a large extent coincides with the area of the Liberty delta system. This region contains numerous isolated salt diapirs and withdrawal basins over which the top of the Yegua has a relief of several thousand feet. Isopach maps indicate that the salt was actively flowing during deposition of the Yegua. This area also was affected by growth faulting, glide plane faulting and other structural features. In contrast, the flanking areas lack evidence for salt deformation, and otherwise display features of normal unstable shelf edges (Winker and Edwards, 1983). All of these areas were disturbed by late movements along growth faults that were most active during Wilcox deposition.

STRATIGRAPHY Biostratigraphy and Age

The Yegua and Cook Mountain formations make up the upper part of the Eocene Claiborne Group (Fig. 3). The Globigerinatheka semiinvoluta foram zone appears in the Moodys Branch Formation of the Jackson Group overlying the Yegua Formation and the Orbulinoides beckmanni foram zone occurs below the lowest sand-bearing units in the Cook Mountain.

The boundary between the middle and late Eocene is taken as the boundary between the Cook Mountain and Yegua Formations. Traditionally, the boundary between the Yegua and Cook Mountain was based on the first downhole record of Ceratobulimina eximia. However this foram occurs at the base of the sand-bearing interval and is environmentally controlled. In deeper water environments downdip, planktonic forams are present. These can be related to the worldwide planktonic foram zones. Recogni- tion of the Truncorotaloides rohri zone at the top of the middle Eocene has been based upon the occurrence of Truncorotaloides rohri and T. topilensis and the acicular forms of Morozovella spinulosa and M. lehneri (Globoro-

talia spinulosa and G. lehneri in industry usage) in addition to Acarinina bullbrooki and A. spinuloinflata (Globorotalia bullbrooki and G. spinuloinflata in industry usage).

In deeper section, a morphological change within Truncorotaloides topilensis has been used by Paleo Control Inc. to further subdivide the T. rohri zone. This is closely associated with Clavulina guayabalensis. The sand-bearing part of the Cook Mountain occurs in the upper Cook Mountain, above this T. topilensis variety and C. guayaba- lensis.

In summary, the Yegua section is included within the G. semiinvoluta zone while the Cook Mountain section is included within the T. rohri zone.

Log Stratigraphy The Yegua-Cook Mountain was subdivided with eight

markers in middle Texas and 12 markers in southeast Texas and southwest Louisiana (Fig. 3). The eight markers in middle Texas approximately coincide with eight of the 12 markers used in the latter area. The markers are generally characterized by low self potential (SP) and low resistivity, indicating the maximum clay content. Examples of logs with markers are shown in Figures 4, 5, 8, 11, and 12. The continuity and distinctive log signatures of these shales contrast markedly with the heterogeneous and highly variable sands and shales that occur between the marker shales.

Most of the approximately 4,500 logs that were used occur on the stable shelf over a distance averaging 30 miles from the shelf edge, and therefore the correlations are comparatively straightforward. However, over long dis- tances small datum changes are usually required. Between markers, SP and resistivity curves were used to estimate lithology. Specifically, the following parameters (Fig. 4) were mapped for each interval: 1, interval isopach; 2, net sand; 3, blocky sand; 4, percent sand; and 5, log facies.

SAND ------- - I

NET SAND ISOPACH

UPPER SHALE MARKER LOG FAClES

r

&

SHALE MARKER

EXAMPLES OF ELECTRIC LOG FACIES CBL CFU

PMB ME

SPL MAR

jll! SPLAY 1115 MARINE

Figure 4. Definitions and examples of parameters mapped from the logs: interval isopach, net sand, blocky sand, percent sand, and log facies. The log facies types includes possible depositional environment.

Houston Geological Society Bullet~n. December 1990 40

The fine-grained marker shales that can be correlated for distance exceeding 100 miles along strike and far up dip represent relative transgressive events of regional extent (Fig. 5). Small datum shifts suggest local interruption by small regressive pulses. The section between the markers represents regressive deposits primarily, including a variety of progradational deltaic to marine sediments. The types of maps described above were used to understand the geometry and orientation of the sand bodies.

Emphasis was also given to the identification of erosion surfaces such a s regional unconformities, local channel downcutting, erosional canyons, gullies, and slumps (Fig. 5), because of their potential role in recognizing depositional sequence boundaries. Erosion surfaces were identified in map view primarily where the marker at the base of the map unit was rernoved (for example, in Fig. 5). Erosion within a map unit was not mapped although erosion could be indirectly inferred from the distribution of blocky sandstone and log f ac~es (for example shown in Fig. 11).

Seismic Stratigraphy The log-based stratigraphic framework is effective on

the stable shelf, but breaks down in the vicinity of the unstable shelf edge due to large-scale erosion, faulting and slumping. In this area of limited well control, seismic data is invaluable in illustrating gross geometry of the strata, and the various types of stratal boundaries. Checkshot surveys were used to tie log stratigraphy, biostratigraphic tops, and dipmeters to seismic lines.

Seismic data (Fig. 6) reinforces the high continuity of log correlation markers on the stable shelf, indicating a topset geometry formed by vertical stacking of sedimentary units. In contrast, large-scale erosion near the unstable shelf edge resulted in anomalously deep water conditions. These areas were filled with inclined strata showing a foreset geometry and are often associated with gravity instability features such a s growth faults, glide plane systems, and rotated slump blocks (Fig. 6). Log correlation in such areas IS difficult, and the transgressive shale markers are unreli-

6 8 10

k k k k k 4 MILES

0

0 1

DIP ANGLES 0.2 ( V . L - 120X)

0.3

Figure 5. Stratigraphic strike sect ion showing continuity of shale markers along strike, variability of deltaic facies between transgressive shale markers, slump-formed erosional unconformities truncating updip shallow-water strata, and contrasting slump fi l l . Log tick marks are 100 feet apart. Location shown o n Figure 2.

4 1 Houston G e o l n y ~ r d S o ~ t r t y B u l l e t ~ n D e c e r n t ~ r r 1990

Figure 6. Portion of seismic line showing complex structure and stratigraphy. Note topset (horizontal) versus foreset (inclined) deposition, relation of depositional style to steep slump-formed uncon- formities (dashed), and presence of faulted strata overlying a glide plane at the top of the slope wedge (from Edwards and Tuttle, 1989b; seismic data originally provided by TGS Onshore Geophysical Company.

able as they change character downslope or are eroded away.

LARGE-SCALE DISCONTINUITIES In the context of an unstable shelf edge, such as that

present in the Yegua-Cook Mountain, large-scale discon- tinuities are represented by two end-member processes that are not always readily distinguished from each other (Mitchum et a/ , 1977; Fig. 7). Stratigraphic discontinuities are a type of erosional unconformity caused by catastrophic mass movements, while structural discontinuities result from sediment failure and downslope creep of sediment masses overlying the failure surface. (Not included here are small-scale local unconformities that may develop beneath fluvial or distributary channels.) The distinction between the two types is drawn on the basis of whether or not the discontinuity is exposed at the depositional surface. Con- sequently the nature of sediment resting on the discontinuity differs in each case. Ideally, unconformities are overlain by undeformed strata, that drape over and may onlap and/or downlap the underlying surface. In contrast, structural discontinuities such as glide planes, are overlain by slightly

deformed strata overlying low angle surfaces, or rotated and faulted blocks of sediment.

Not as easily interpreted as the end-member cases described above are cases where: 1, unconformities are overlain by rotated slump blocks (thus mimicking a glide plane); and 2, low-angle glide planes either are overlain by truncated strata that resemble baselap, or are overlain by severely deformed strata that resemble slumped sediment (thus mimicking an unconformity).

In contrast to the regionally extensive unconformities that occur at the shelf edge, unconformities on the shelf can be mapped at best only locally, usually having a narrow dip-oriented geometry, and in association with a basal sandy fill.

Unconformities Two types of unconformities generated by mass

movement are recognized in the Yegua: slumps and gullies. They are distinguished on the basis of geometry, size and fill.

Slumps are variable in size, but are generally strike- elongate. They truncate from several hundred to several thousand feet of older section, and extend along strike from around ten to more than one hundred miles. In the Yegua Cook Mountain section, slumps range in age from upper- most Cook Mountain to lower Yegua (Figs. 2,3, and 5), and large-scale slumping also affected older section within the Claiborne Group. The slumps are filled with a variety of deposits including backward-rotated slump blocks, on- lapping strata (slump head region), downlapping strata (toe region), undeformed to growth-faulted concordant strata (basin floor), and chaotically bedded slump deposits. In some cases onlapping deposits contain productive sand- stone reservoirs (Fig. 8). Some slump-formed erosion surfaces truncate Wilcox growth faults.

Gullies are narrow, dip-oriented erosional features that are filled primarily with slumps or fine-grained onlapping mud drape. Gullies are formed by headward erosion and slumping of poorly consolidated sediment near the shelf- edge (Steffens, 1986). In contrast to submarine canyons, which are much larger, gullies did not serve as channels for funnelling large quantities of river-derived sediment across the shelf to the slope and basin floor. Submarine canyons, large-scale dip-oriented erosional features, have not been recognized in the Yegua-Cook Mountain section in the study area.

Glide Planes Glide plane fault systems are typical of the Yegua-Cook

Mountain section. Glide plane systems commonly affect up to several thousand feet of section. Claiborne-age fault systems are typically offset by older Wilcox faults that continue to propagate upwards. In the major systems observed, the glide plane most commonly originated as a major unconformity, and also formed above wedges of slope sediment deposited on unconformities (Figs. 6 and 7). In other cases the factors controlling the location of the glide plane could not be determined. Fanning of dip within a fault block and contrasting seismic reflectors in adjacent fault blocks are used to establish faulting contemporaneous with sedimentation.

In addition to the large-scale glide plane systems described, small-scale "shallow glide plane systems" were also observed using seismic, log, dipmeter and micro- paleontologic data. These systems (Fig. 9) affect less than a

Houston Geo log~ca l Society Bulletbn. December 1990

Figure 7. Sequential schematic representation of development of unstable shelf-edge discontinuities. A, original undeformed horizontal s trata with location of incipient failure. B, unconformity created by removal of sediment by slumping. C, patches of deformed slump blocks may litter t he erosion surface. D, abrupt break in slope and relative deepwater conditions cradle slope wedge of onlapping sands o r condensed section muds. E, progradation of shallow water deposits resume t o fill slump scar. F, s teep head of slump scar may favor development of gravity instability features above slope wedge; positions of incipient growth faults a r e shown. G, growth faults form in upper part of slump scar; underlying condensed section serves a s glide plane. H, glide plane fault systems (shallow example shown here) develop on the steeply-dipping prograding forests. The glide plane may b e condensed section in the foreset.

thousand feet of section, developed very rapidly by down- slope sediment creep and sediment loading, and formed the locus for ve ry rapid sedimentation due to the high local subsidence rates. Bathymetric lows can be filled by gravity flow deposits. The resulting features can be difficult to resolve on seismic data due to the small scale.

Slump-formed unconformities and glide plane systems are an ~mpor tan t part of the Yegua-Cook Mountain deposi- tional system and their recognition is vital to unraveling the downdip stratigraphic framework. Both features can lead to the development of stratal patterns that in dip lines resemble sequence stratigraphic features such as truncation beneath unconformlties and downlap of lowstand systems tracts.

DEPOSITIONAL ENVIRONMENTS Depositional environments cf units within the Yegua-

Cook Mountain were determined by detailed mapping (see log stratigraphy). The unpublished maps (Edwards and Tuttle, 1985, 1987, 1989a) show the positions of inferred major delta systems, distributary channel-mouth bar com-

plexes, fringes of mouth bars, interdistributary bays, iso- lated and incised channels and strandplains (Fig. 10). Other data was used in addition to infer environments onto the slope.

There is a broad change from proximal to distal deltaic environments from updip to downdip across the study area, and a general increase of wave-dominated environments along strike toward the delta flanks (Fig. 10). A very prominent feature is the mud-dominated shelf, primarily in the flank areas, that separates updip from downdip sand units.

Maps and cross sections reveal sand-filled channels that can be projected downdip across the shelf t o equivalent sands at the shelf edge (Figs. 10 and 11). The continuity of bounding markers bracketing these sands and the absence of other mapped potential sources for the downdip sands suppclrt the interpretation. Rarely, in updip areas, channel sands truncate the underlying marker. Across the mud shelf, the absence of mouth bar deposits associated with the channels is noteworthy, and serves to contrast these

43 Houston Geological Society Builetln December 1990

s h A 1 l o r dlp dlvorqenco 1

Figure 9. Shallow glide plane systems. These result in rapid, highly localized subsidence rates in the head region. Bathymetric lows and s t eep slopes favor deposition of gravity flow deposi ts . Correlation between fault blocks is excellent in pre-glide s trata , but very poor in post-glide s t ra ta until the s t ruc t re stabilizes. These can typically involve packages less than 1,000' thick, and may not be readily apparent on seismic data, especially away from the head region.

suggest that shelf-edge progradation was very complex, with alternation of deposition, erosion and bypass (Fig. 12).

Micropaleontological data suggest inner to middle neritic conditions for the middle to outer parts of the stable shelf, with outer neritic to bathyal environments character-

izing downdip zones. Neritic faunas in basinal positions are inferred to have been retransported from the basin margin.

CYCLICITY IN THE YEGUA-COOK MOUNTAIN Several scales of depositional and erosional cycles can

be recognized in the Yegua-Cook Mountain interval (Fig. 3). At the largest scale is the entire Yegua-Cook Mountain sandy clastic interval. This is referred to here as an A order cycle. The next finer scale is that of the regionally corre- latable shale markers. These are more extensive than the erosion surfaces formed by shelf-edge slumping, and are referred to here as B order cycles. Depending on the interpreter, there may be from ten to twenty such markers in the Yegua-Cook Mountain. Lastly, the finest order cycles are represented by small-scale depositional units that occur between the B order markers. and which extend for onlv a few tens of miles along strike. These are referred to here-as C order cycles. The designation of some units as either B order or C order cvcles is subiective.

The appearance of the Border cycles is similar to that of parasequences (Fig. 7, Van Wagoner et a/., 1990), but thev also exhibit some of the features of seauences. Similar cycles in the Miocene of Louisiana were interpreted as high-frequency sequences (Van Wagoner et al., 1990).

As discussed below, the B order cycles are interpreted to be similar to fourth order cycles reflecting allocyclic eustatic events having a periodicity of hundreds of thousands of years (Van Wagoner et a/., 1990), whereas the C order cycles are interpreted to be local autocyclic events. The large scale A order event is presumed to reflect a tectonically controlled influx of coarse-grained terrigenous sediment to the coastal plain.

Boundaries between cycles can also be placed along erosion surfaces. The Yegua-Cook Mountain has two main types of erosion surfaces: locally developed axes of channel incision on the shelf, and local to regional scale slump- formed erosion surfaces along the outer shelf and slope. Evidence for channel incision and bypassing of sediment across a muddy shelf to the shelf edge occurs approximately

T E R

Figure 10. Simplified paleoenvironmental map of two adjacent B order cycles in the Yegua-Cook Mountain interval. Central Liberty delta system depocenter has closely spaced distributary systems and coalesced mouth ba r s (MB) that extend ou t t o t he shelf edge. Along the delta flanks, distal mouth bars (DMB) d o not reach the shelf edge, but isolated channels appear t o be related t o documented downdip sandstones. The approximate location of the contemporaneous shelf edge is shown.

4 5 Houston G e o l o ~ r a l S o c r t y Bu l l e t~n De~ernber 1990

12 times in the Yegua-Cook Mountain interval (Fig. 3). Evidence for shelf-edge slumping was observed at eight stratigraphic levels in this interval, although more data from downdip areas could bring to light additional examples. The fact that unconformities cannot by mapped regionally at this time precludes their use a s markers for stratigraphic mapping. Furthermore, because the unconformities feather out in the shale to sand transition formed by progradation (Figs. 5 and 8 ) , their correlative conformities cannot be readily mapped with well data.

Biostratigraphic zonations using benthonic and plank^ tonic forams provide a resolution that is intermediate between A order and B order cycles. Maximum resolution is achieved in the vicinity of the shelf edge, decreasing both toward the shelf and toward the basin.

SEA-LEVEL FALLS IN THE YEGUA-COOK MOUNTAIN

Two lines of evidence suggest that falls in sea level occurred repeatedly during the Yegua-Cook Mountain interval. Both of these relate to facies relationships in a dip direction along the delta flanks.

Downdip displacement. Within an individual mapping unit, shallow water deltaic facies are present both updip on the shelf and downdip at the shelf edge, separated by a mud zone in between. Potential reservoir facies along the shelf edge are highly variable, including coarse-grained gravity flow sandstones, laminated fine-grained storm deposits, and medium grained high-energy tractional deposits, based on interpretation of whole cores. In several cases, it has not been possible to map with logs the channels through which sediment was bypassed across the shelf to the shelf edge, although they are occasionally visible on specially processed seismic data (Allen, 1989).

The downdip displacement (or "basinward shift") of deltaic facies is inferred to be a resDonse to a sea level fall that exceeded the rate of shelf subsidence. As a result, distributary channels became incised, and sediment was bypassed to the shelf edge. Detectable erosion in updip areas was limited to the channel incision. Due to the paucity of well cont:ol, and the relatively small area affected by the channcis, they are rarely encountered in the muddy shelf area.

Incised channels. Narrow, linear, dip-oriented ero- sional features can be mapped both in updip and downdip portions of the shelf. Such features usually d o not erode down into the underlying mapping horizon. They may be filled with a variety of facies including blocky sands, fining upward sands and shale. In some cases, the channels can be mapped with enough control to project them dowdip to known shelf edge sand accumulations. In other cases, downdip wells have not yet been drilled to test the downdip nroiections. . ,

Incised channels are most obvious where they are surrounded by shelf muds, a s opposed to the normal sandy deltaic facies. It is inferred that a sea level fall resulted in channel incis~on and bypass of coarse sediment through the channels rather than denosition of normal mouth bars around the channels.

In some cases. both incised channels and disnlaced deltaic facies can be mapped updip from a shelf edge sand body (Fig. 11)

According to Posamentier and Vail (19881, fluvial incision, sedimentary bypass of the shelf, and abrupt basinward shift of facies characterize type 1 unconformities caused by sea-level fall at a rate exceeding the rate of shelf subsidence. The other characteristics of type 1 unconform- ities, stream rejuvenation and coastal onlap, could not be evaluated with the available data.

Incised channels rather than submarine canyons are thought to be the conduits through which coarse sediment was funneled to the slope and basin floor. Large-scale slumps a re envisioned more a s sediment traps for slope sands than environments for the bypass of sediment further down the slope into the basin.

BOUNDARIES FOR REGIONAL MAPPING OF DEPOSITIONAL UNITS

The documentation of sea-level falls occurring between transgressive marine shales suggests that, according to depositional sequence theory, a sequence boundary must exist within this zone. While well data readily permits the identification of erosion surfaces such as incised channels on the shelf and slumps on the slope, the sequence boundary cannot be detected, but inferred for most wells. For example, in the south Louisiana Miocene where the highstand deposits below the sequence boundary are very shaly, the sequence boundary was made coincident with the overlying flooding surface (transgressive shale) (Fig. 22. Van Wagoner et al., 1990). Where the highstand deposits are sandy or heterogeneous, tracing sequence boundaries between wells appears to be forced, compared to tracing flooding surfaces (Fig. 33, Van Wagoner et a/, 1990).

Figure 11. Stratigraphic dip section showing updip (stable shelf) and downdip (unstable shelf edge) sands with intervening muddy shelf, Wharton delta flank. Whole cores through the downdip wells suggest deposition by sediment gravity flows. Wells have been selected to show best sandstone development. Inter- pretive lines on the section show erosive base of blocky channel sandstones and margins of mouth bar sandbodies. Productive wells downdip are in Toro Grande Field. Log tick marks are 100 feet apart. Location shown on Figure 2.

Figure 12. Stratigraphic dip section through updip margin of shelf-edge slump f i l l showing change from flat appearance of topsets to inclined appearance of foreset deposits, and complexity of the boundary zone. Note occurrence of sands in vicinity of topset- foreset boundary near inferred contemporaneous shelf edge. Some of these sands have no adjacent equivalents in nearby topset strata on the shelf. The change from highly correlatable strata in the topsets to poorly correlatable strata in the foresets is easily noticeable. Log tick marks are 100 feet apart. Location shown on Figure 2.

Houston Geologfcal Soc~ety Bulletfn December 1990

DAWU: WALE MARKER

U P W DLLTAS ON W S € D M W L S STABLE SHELF SEOHYUT B W A S S ON s T m E SHELF ACROSS W L F

0 OOWOIP DISPLACEMENT Of DELTAIC FACES

0.5

1

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DIP ANGLES - EXPANYON

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10 5 3 2

Figure 12.

UPDlP DOWNMP

0 G a 0 0 a 0 0 0 0 OIUY PUU u m

COMPLEX PROGRADATIONAL BASIN FILL

In the Yegua, significant erosion on the shelf was limited to incised channels. and these channels occasionally occur in close association with and sometimes pass updip into channelized delta front deposits. Apart from the incised channels, high stand deposits were preserved, and not removed a s sea level fell. Nevertheless, for the purpose of predicting downdip sands, it is important to map erosively- based channel sand bodies separately from other sand bodies in order to establish the presence and orientation of potential sources of coarse sediment to the shelf edge and basin.

The relationship between the regional slump-formed unconformities and euctacy remains unclear. In general, the erosion surfaces appear to terminate in the progradational part of the B order cycles, suggesting that they formed during a fall rather than a rise in relative sea level. Triggering of slumps would more likely occur when deposition was focused at the shelf edse rather than on the stable shelf. - However, these unconformities d o not appear to be geo- graphically related to the local erosion surfaces developed beneath incised channels on the shelf. Rather, they appear to be associated with the regional large-scale progradation of the shelf edge and slope (A order cycle) of the Liberty delta system (see Figs. 2 and 10).

EUSTACY AND HIERARCHIES OF CYCLES

The 1-2 m.y. time during which the Yegua-Cook Mountain interval was denosited was dominated bv the Middle-Upper Eocene type 1 sequence boundary, and the early part of this interval was possibly associated with a type 2 minor sequence boundary (Haq e t a/., 1988). The fre- quency and magnitude of these cycles suggests, at best, a tenuous relationship to the large scale A order cycle of the Yegua-Cook Mountain interval. In view of the enormous stratigraphic complexity of this interval, the connection to the present version of the global eustatic curve is of little help in attempting to predict downdip sands.

The frequency of B order cycles, in association with evidence for sea level fall and numerous shelf-edge failures indicates that stratigraphic analysis and downdip sand prediction must be at least of the B order scale to be effective. In what manner is the B order scale related to depositional sequences? The B order cycles, with their downdin disnlacement of facies. incised channels. con- densed'section, and occasional 'shelf-edge slumps: have many of the attributes of depositional sequences. Using the available biostratigraphic resolution, it appears that the cycles were formed on the order of approximately five to 10 per MA and are at the high frequencey end of the duration of sequences (table 1, Van Wagoner et a/ . , 1990).

Eustatic rises in sea level have been invoked as causing the deposition of condensed section in environments ranging from shelf (Coleman and Roberts, 1988) through slope (Armentrout, 1987) to abyssal plain (Weimer, 1989) environments, by leading to a sharply reduced supply of terrigenous sediment to the basin. These condensed zones, where nreserved from erosion o r deformation. could be used as time markers and depositional cycle boundaries having a broad regional distribution. Possible condensed sections have been observed in the downdip Yegua, but the lack of sufficient micropaleontological resolution precludes shelf to deepwater correlation at this time.

HYDROCARBON PLAYS IN DOWNDIP SANDSTONES

Early downdip production in the Yegua was from expanded, growth-faulted section in the depocenter and occasional onlapping sands in slump-formed depressions on the slope. Recent discoveries in the downdip Yegua trend have included lowstand shelf-edge deltas and highly com- plex dip-oriented slope fans. Deeply-incised channels near the shelf edge have locally been very prolific. Deepwater turbidites systems have been drilled and cored, but have not yet yielded commercial production.

CONCLUSIONS

1. In unstable shelf edge settings, stratal patterns significant to "depositional sequence" analysis can be mimicked or modified by processes such as growth faulting, erosion by mass wasting and sediment sliding and defor- mation, complicating the procedures of sequence strati- graphic analysis.

2. Detailed regional mapping for the prediction of shelf margin and slope sands is more effective if stratigraphic units are bounded by transgressive marine shales ("con- densed section" or "flooding surfaces") rather than by unconfo rmi t i e s a n d thei r co r re l a t ive confo rmi t i e s ("sequence boundaries"). Other factors such a s paleo- bathymetry, contemporaneous deformation ,and salt movement must also be considered for reservoir predition.

3. The Yegua-Cook Mountain interval has three orders of cyclicity. Compared to the other cycles, the intermediate Border cycle displays more of the features associated with "depositional sequences." (e.g., Van Wagoner ef al, 1990). They may represent high frequency sequences that formed at a rate of 5-10 per m.y., comparable in duration to "fourth order eustatic cycles."

4. Sea level fall has been documented by identifying abrupt basinward shifts in facies and channel incision. These features are more easily recognized along the delta flanks than in the depocenter.

5. Large-scale unconformities were observed almost exclusively at the shelf edge and slope. These were formed by mass movement, and the resulting slump scarsare not a s extensive regionally a s Border transgressive shale markers. O n the shelf, widespread mappable unconformities could be observed in neither logs nor seismic data.

6. Certain relationships observed in the Yegua-Cook Mountain, such a s the tendency for shelf-edge mass failures (slumps or embayments) to occur in the depocenter, and for effects of base level falls to be more discernable along the delta flanks, may be applicable to other shelf margin trends.

ACKNOWLEGMENTS The proprietary studies (Edwards and Tuttle, 1985,

1987, 1989a) from which this report has been extracted were underwritten primarily by oil and gas exploration companies whose support is gratefully appreciated. The studies were prepared in association with J . Loyd Tuttle and colleagues at Paleo Control Inc. who carried out all of the micropaleontological analysis. In addition, T G S Onshore Geophysical Company provided access to much of their seismic data in the study area. Execution of the studies benefited from the assistance of Diane Frossard, Rebecca Brown, Duff Stump and Sharon Jones. I am indebted to

Houston Geological Society Bu l l e t~n December 1990 48

Charlie Winker. Jnry Pacht and Boh Raynolds for their constructwe and insightlul reviews of two drafts o l this article.

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MARC 6. EDWARDS-Biographical Sketch Marc has been a con-

sul~gmlogist since 1982. During this time he has investigated theapplication of mkrowrnputers to geo- losic arapping, and has applied the results to &onat geok&A studies of the Yegua Foranation in Texas and Louisiana.

Prior experience in- cludes studies of seo- pressuredgeothermal re- sources in the Wllcox and Frio of South T e w whXe at the Bureau of E c d

l%D. .in sediment& from Oxford. .He w6wd the AAPG Sproule Mewrial Award in I983 for the AAPG Bulletin article "Upper WilcoxRosita delta bvaem of South Texw:grwth-faultdshelf-edg dettss". Ma@nstructs the AAPG two;day workshop on "Stratigraphic Analysis 01 Growth-facilted ResiomUsing WeU LC&, andbansssoci- ateditor tor the AAF'G&Ilkthr.'He is amember of MPG, SEPM, GSA, HGS, and lAS,

Narc is cuneatlylaunchit@a new stwdy of the Miocene

-. - -~

G e d w in Austin, and investigations of the Earents Wf and Spitsbergen while at the Norwegian Pok Institute in '

Oslo and the Norwegian Continental Shelf lnatihrte in Trondhobn.

Marc received hiB.S. h Geology at C.C.N.Y. and his i

of South Louisiana and is interested in coming into contact withexplorationistsaaive in thiiarea. Marc can be reached at 7284215 or 668.5488 (office).