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DHI's (Direct Hydrocarbon Indicators)

Part 1: Simple Seismic Attributes

INTRODUCTION

Direct Hydrocarbon Indicators, or DHI's (also commonly known as, HIs

!Hydroarbon Indicators"), are used e#ery day in t$e oil and %as industry by

tec$nical pro&essionals to $elp searc$ &or $ard to &ind, or simply o#erseen,

$ydrocarbon deposits $ere are many &actors t$at must be taken into

consideration to apply seismic attributes properly to name a &ew: 1) understandin%

rock p$ysics *) t$e si%nal to noise ratio o& t$e seismic sur#ey +) poor processin%

) insu&&icient (or erroneous) ac-uisition parameters, and so on

.$en %as or oil replace t$e interstitial brine water in e/istin% reser#oirs, t$e seismic

re&lection coe&&icient in$erently c$an%es (t$is is mani&ested by a di&&erence in t$e

acoustic impedance o& t$e $ydrocarbon0bearin% one as compared to t$e brine0

saturated one eit$er abo#e or below t$e reser#oir in -uestion) $is work must be

done in order to determine i& a well s$ould be drilled 23 42 D3I556D 0 t$is cannot

be stressed enou%$ t$ere&ore DHI's are a cornerstone application &or t$e $unt o& oil

and %as deposits It s$ould be noted t$at not all seismic anomalies are DHI7s, and

not all DHI7s are o& e-ual -uality amplitudes can be caused by &actors not related to

$ydrocarbon accumulations 0 t$is is a key point t$at many %eop$ysicists commonly

o#erlook (t$is will be discussed in %reater detail in Part II o& t$is write0up)

Potential Pitfalls in DHI Interpretations

$e relative amplitude #alue can be deteriorated and8or altered dramatically by

post0processin% o& t$e seismic data $at is, t$e numbers associated wit$ t$e

seismic amplitude #alues are unitless, t$ere&ore, t$ey &all into no particular unit

sc$eme (t$ere is a lar%e misconception in t$e oil industry t$at all seismic

amplitudes &it nicely into a 01*9 to 1*9 #alue sc$eme 0 HIS IS 42 S2 0 in &act,

t$ey can ran%e anyw$ere &rom 0;;;*+ to ;;;*+ to as $i%$ as 01

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Durin% processin%, erroneous arti&acts may mani&est t$emsel#es in t$e seismic

dataset 0 t$ese need to be studied wit$ %reat detail to be certain t$at t$ey are

indeed real or not real an error o& t$is ma%nitude could cost a company >1;;'s o&

??@SD's, so it is #ery important to understand w$at one is doin% and i& t$ey are

%oin% about it t$e proper way $ere&ore, analo% data, calibrated well lo%s, etc,

must be used to $elp determine i& t$ese arti&acts are true representations o&

$ydrocarbon deposits

2t$er maor issues include t$e calibration o& wells t$at are not in t$e same %eolo%ic

pro#ince because t$ey come &rom a di&&erent en#ironment o& deposition, or t$e

wells used to calibrate t$e data are simply too &ar away &rom t$e well bein% studied

Additionally, i& t$in%s suc$ as strati%rap$ic c$an%es (e%, %oin% &rom a c$annel &ill

sandstone en#ironment to a t$in bedded le#ee en#ironment) t$e well ties and t$eprospects will be wildly di&&erent And per$aps t$e most common error is t$e lack o&

su&&icient inte%ration o& %eolo%ical data8interpretations &or t$e prospect bein%

e/amined

Basic Seismic Attributes Definitions

Bright spots: 5ocal increase in amplitude on a seismic section (presumably caused

by a $ydrocarbon accumulation)

DHI or HCI: ?easurement w$ic$ indicates t$e presence or absence o& a $ydrocarbon

accumulation (bri%$t spot, dim spot, &lat spot, s$adow one, tc)

Phase (Polarity Cahnge): Seismic peak c$an%es to a trou%$ (or #ice #ersa)

Dim Spot: 5ocal decrease in re&lection amplitude, %enerally occurs in low porosity

sands (1;B to 1=B)

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Ci%ure ;1: Clat Spot s$owin% &luid contact o& a %as &ield wit$ an underlyin% water le%

(blue0das$ed line)

Ci%ure ;*: as re&lections &rom t$e 4ile Delta in 6%ypt t$e $i%$ amplitude red

re&lection (trou%$) is &rom t$e top o& t$e %as in t$is anti&ormal trap con#ersely, t$e

$i%$ amplitude blue re&lection is &rom t$e base o& t$e %as, or &rom t$e &luid contact