3D Gravity Modeling of Osage County Oklahoma for 3D Gravity Interpretation

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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.
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  • 1. 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
  • 2. 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.
  • 3. 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.
  • 4. 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.
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