Abstract: Modern Analytical Techniques for Fault Surface Seal Analysis: A Gulf Coast Case History

Mary J. Broussard, Brian E. Lock

Today's exploration methods, with a major reliance on mapping with 3-D seismic data, generate a great deal of potential information about prospective reservoirs. Effective prospect evaluation requires consideration of the sealing characteristics of faults, and techniques have been developed to improve fault surface analysis. "Allan" fault surface profiles permit assessment of sand juxtaposition across the fault, and can be prepared by manual mapping methods if adequate structural maps are available from 3-D seismic interpretation. Commercial software is available to perform similar analysis directly from the 3-D data.

Marathon's Vermilion Block 331 field was selected for a pilot study. The field consists of a low relief anticline, downthrown to a regional growth fault. Numerous small faults cross the crest of the anticline and compartmentalize reservoir sands of Trimosina A (Pleistocene), Angulogerina B (Pleistocene), and Lenticulina (Miocene) age. Faulted reservoirs with multiple, stacked sands are particularly prone to loss of hydrocarbons by leakage across fault surfaces, so that this field was considered ideal for testing the effectiveness of fault surface analysis. Seal risk was evaluated by means of fault surface profiles along seven of the crestal faults, to determine the limits of trapping potential and paths for vertical migration. Actual hydrocarbon distribution was compared with the predictions made from fault surface analysis.

The role of faults in permitting up-fault migration along the fracture surface or in providing shale smear barriers to cross-fault migration from sand to sand may confound interpretations based only on fault surface profile geometries. For this reason, smear gouge ratios were also determined and used in assessing trapping potentials.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana