3D Gravity Modeling of Osage County, Oklahoma, 3D Geology Interpretation Kevin Crain* and G. Randy Keller, University of Oklahoma, College of Earth and Energy

Summary

New exploration challenges and current research demands

3D gravity modeling with 3D geology interpretations. In

the near future, multi-parameter and multi-dimensional

interpretations represent the observed and expected in situ

geology, geophysical, and petro-physical data that will be

used for join multi-parameter, multi-dimensional

inversions. We present an initial 3D gravity model of

Osage County in northeastern Oklahoma, where there is a

greater than 40 mGal, 100 km diameter semi-circular

gravity anomaly that cannot be effectively removed by

traditional gravity processing techniques.

Figure 1: Index map of the northeast Oklahoma region showing the

location of Osage County and the main structure and geologic

provinces in the region.

Introduction

The large scale gravity anomaly at Osage County, OK,

makes near surface gravity interpretations problematic. The

goal of this gravity model is to enhance the signal of the

near surface geology above the igneous basement by

minimizing the signature of an expected deep sourced

gravity field in the observed gravity. This gravity model is

the result of a density inversion of spatially distributed

observed Free-air gravity and the gravity effect of a causal

geology-constrained 3D interpretation of observed and

expected model geology constructions. Individual

components of the 3D geology interpretation can easily be

modified and updated at any time to address the residual

gravity anomaly.

The Free-air gravity is a non-geology corrected observation

of the instantaneous "local" density distribution. Therefore

to model the Free-air gravity it is necessary to build

geologically constrained and sufficiently detailed 3D

geology interpretations that represent the necessary

complexity of the Earth while allowing for ease of

calculating the model gravity field and geology

interpretation from the Earth’s topographic surface to an

arbitrary depth. We believe a multi-component 3D gravity

model of a complex 3D geology interpretation is preferable

to single density complete Bouguer and terrain corrections

and traditional 2D profile gravity modeling.

Figure 2: Observed Free-air Gravity, [mGal], Osage County, OK.

The estimated Free-air gravity model is the result of a

density inversion of spatially distributed observed Free-air

gravity and the gravity effect of a causal geology-

constrained 3D interpretation of observed and expected

geology interpretation.

The residual Free-air anomaly (RFAA) is the difference

between the observed Free-air and the estimated Free-air

gravity. The RFAA is similar to the complete Bouguer

gravity in that it reflects the unmodeled densities, i.e.,

geologies, but unlike the complete Bouguer gravity, there is

an updatable geology interpretation directly associated with

the RFAA, not the assumed geology of corrections.

The Geology Interpretation

This Osage County geology interpretation assumes simple

“layer cake” geology with a prismatic density structure in

3D Gravity Modeling of Osage County, Oklahoma, 3D Geology Interpretation

the deep crust. The deep crust model is set up to address the

majority of the greater than 40 mGal gravity anomaly and,

at the same time, enhance the near surface geology's

gravity effect.

The components of the interpretation are:

1. Topographic surface and geology extending

almost two degrees beyond the boundary of the

Free-air gravity data

2. An expected igneous basement topography and

geology constrained using drillhole intercepts and

expected topography along with its expected

geology

3. A regular density distribution within 0.10 degree

x 0.10 degree prisms from 25 km to 45 km depths

4. The expected density of each layer and prism is

developed to address the two goals of the gravity

model

Goals:

1. Attempt to model the majority of the 40 mGal

anomaly.

2. Enhance the geology effect of the near surface

and basement geology.

Figure 3: Perspective view of the igneous basement topographic

surface.

The basic density structure of the geology interpretation is:

1. Sediment above the igneous basement

2. Igneous basement

3. Upper crust

4. Lower crust

Figure 4: Expected igneous basement rock types in north central

Oklahoma, based on core analysis. Contours are observed Free-air

gravity. Source: OGS Cir-84.

Figure 5: Perspective view of the surface topography and both the

25 km and 45 km below sea level topography and density

distribution interpretations.

Figure 6: The density distribution of the deep crust, where each of

the interior cells are 0.10 x 0.10 degree on a side and extend 20

km, from 25 to 45 km below sea level.

3D Gravity Modeling of Osage County, Oklahoma, 3D Geology Interpretation

The density distribution of the geology interpretation is:

1. Sediment 2.70 g/cc

2. Igneous basement 2.67 g/cc

3. Lower crust 2.85 to 3.00 g/cc

We want to enhance the gravity signature of the complex

geology of the sediment and basement unconformity

surface. Therefore, we did not include any initial

interpretations and will update the geologic interpretation

in future revisions to address the gravity anomalies.

From Figure 4, it would seem the igneous basement has a

complex geology; though the analysis of the bulk density of

numerous core samples across northeastern Oklahoma

returns a 2.67 g/cc density for the granitic and rhyolitic

rocks in the igneous basement (Denison, 1981).

Results

The estimated Free-air gravity, Figure: 8, is the result of a

density inversion of a 3D geology interpretation. The

difference between the observed and estimated Free-air

gravity is the RFAA gravity.

To evaluate the results, we examined the gravity anomalies

in Figure 7 to Figure 11.

Figure 7: Observed Free-air Gravity, [mGal], Osage County, OK.

The level of complexity of the RFAA in Figure: 9 reflect

multiple sources of gravity signature:

Figure 8: Estimated Free-air gravity, [mGal], Osage County, OK.

1. basement structures

2. low density sand structures within the

sediments

3. possibly a mid-crust igneous body

Figure 9: Residual Free-air anomaly, RFAA, [mGal], reds

indicates too high of a density, blues reflect too low of a density.

3D Gravity Modeling of Osage County, Oklahoma, 3D Geology Interpretation

To illustrate the sandstone and basement structures, Figure

9 shows correlation to the Lower Red Fork Sands and

known basement faulting (Andrews, 1997).

Figure 10: An extracted overlay of Plate One of OGS Special

Report SP-97-1 (1997) over the RFAA.

Figure 11: Vitrinite reflectance of organic matter. The vitrinite

reflectance value shows a strong correlation with maximum burial

temperature.

The third point, a mid-crust high density igneous intrusion;

the remaining nine mGal low reflects the need to increase

the density in the upper crust. To support this hypothesis of

a possible upper crust intrusion occurred is in the vitrinite

reflectance values (B. Cardott, personal communication,

2011), Figure 11. The data show an increase in the two

reflectance values of Devonian age shale on the southern

edge of the gravity low in the center of the RFAA map.

Work is ongoing to expand the vitrinite reflectance

measurements northward.

Conclusions

Gravity modeling answers more questions if you pose the

question in the form of hypothesis testing. These results are

for a single and first pass 3D gravity model using a simple

3D geology interpretation. The result is an estimated 3D

residual Free-air gravity field that shows geologically

consistent gravity signatures. That will be addressed in

updated geology interpretations. Each of the individual

components of the Osage County, OK, geology

interpretation was built to test one or more geologic

hypothesis. For example, by building a smooth igneous

basement unconformity and simple uniform sediment

geology, when it is known that both are geologically and

structurally complex, the residual Free-air anomaly reflects

their complexity. Then, by updating one of the components

of the geology interpretation, the validity of that individual

component can be tested. The next change to the geology

interpretation will update the upper crust to include an

igneous intrusion addressing the remaining gravity high

(blue in color) near the center of the survey area. An upper

crust intrusion 10 to 15 km below sea level could also

address the unexpected high vitrinite reflectance values at

the south end of the same gravity high.