Jin-wook Kim1, Robert R. Berg1, Joel S. Watkins1, Thomas T. Tieh1
(1) Texas A&M University, College Station, TX
ABSTRACT: Trapping Capacity of Faults in the Eocene Yegua Formation, East Sour Lake Field, Southeast Texas
The nature and fluid-flow properties of faults in the Eocene Yegua Formation were investigated to evaluate the hydrocarbon entrapment and fault-seal risk. A core of Yegua sandstone, 45 ft thick at a depth of 9939 ft shows highly sheared bedding and increasing dip from 10 to 30 degrees with depth. Porosity and permeability of the sheared zone ranges 10 - 12 % and 1 - 5 md, respectively. A reduction of porosity and permeability with depth approaching the sheared zones created the permeability barrier that prevents hydrocarbon drainage and compartmentalized the reservoir. Fractures less than 6 µm in width were also observed in sheared zone, indicating the increase of local permeability, which may reduce the capillary pressures. Most pores in this sandstone are of secondary origin. In sheared zones pores were collapsed and deformed due to the pressure solution and shear stress. Physical characters of sheared zones were visualized by optical and Scanning Electron Microscopy (SEM). Initial oil-water capillary displacement pressure of the sheared zone in fault is 100 psi, sufficient to trap an average oil column of 680 ft. However, the magnitude of sealing capacity may depend on the composition of sediments, and the distribution of faults and the fluid-flow properties of fault rock.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado