Modern seismic attributes enhance subtle features in seismic data that help delineate fractures, faults, and diagenetic features. Recent attribute analysis of Dickman Field, Ness County, Kansas has delineated two sets of fractures that are most easily seen on most-negative curvature volumes and have been spatially correlated to production of water and oil in the Mississippian carbonates. Several of these karst-enhanced fractures are filled with the overlying Pennsylvanian Age Cherokee Shale, thereby forming lateral seals. Others form conduits to the deeper aquifer.
In Dickman Field, the temporal thickness of the valley-shaped lineaments is between 2 and 4 ms. Since these lineaments are seen on seismic data rather than on logs, it needs to be determined if the ‘depth’ is due to karstification and dissolution of the Mississippian carbonate or due to a velocity ‘push-down’ effect. Alternative overburden velocity push-down hypotheses include fractures include stress release through fracturing, diagenetic alteration resulting in porosity enhancement, gas charged fractures from below traveling along fractured zones of weakness, and porosity preservation through hydrocarbon charge.
The objective of this thesis is to validate which of these hypotheses are consistent with the seismic experiment. To this end, a suite of 2D elastic models has been constructed based on velocities, densities, porosities, and depths corresponding to Dickman field. 2D seismic lines were then numerically acquired along each of these alternative models using a fully elastic wave equation finite difference algorithm. Each numerical seismic line was then processed through prestack depth migration and evaluated whether they corroborate the measured 2- to 4-ms depth anomalies.
Results of the first several models indicate that the valley-shaped anomalies are due to diagenetic dissolution and collapse rather than due to velocity push-down. While the two hypotheses are equally valid for vertical convolutional models, most of the finite-offset ray paths in our wave equation numerical seismic gathers do not travel through the low velocity zone. When migrated and stacked, the perturbation of the hypothesized low velocity zone impacts only a few traces in the gathers. The karst features, in effect, should be difficult to image in the seismic, but are visible in the collected 3D seismic.
Dickman Field lies just west of the Central Kansas Uplift in Ness County, Kansas. Producing from a Mississippian reservoir, Dickman Field was discovered in 1962 by Grand Mesa Operating Company and has since produced approximately 1.6 million barrels of oil. Sequestration efforts for the depleting reservoir have opened up a thorough review of the Dickman Field in order to evaluate its credentials. 3D Seismic was shot and analyzed alongside well log data. Attributes were generated by Kurt Marfurt to help delineate the subtle lineaments. My goal in this study is aimed at furthering the seismic analysis of the karst features through forward modeling. The equations formulated by Freznel and Widess have been consulted in preliminary model generation in relation to resolution, but resolution is not my objective in this study. The dissolution features can be as small as 1-2 milliseconds on the seismic traces; too small to be realistically resolved with existing technology.
