3d gravity modeling of osage county oklahoma for 3d gravity interpretation
TRANSCRIPT
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.