McMANUS, KATHLEEN M., and SCOTT D. TIBBETTS, Exxon Co. USA; and PENNY E. PATTERSON, Exxon Production Research Co., Houston, TX

ABSTRACT: Sequence Stratigraphic and Depositional Controls on Reservoir Quality of Upper Jurassic Cotton Valley Sandstones, Carthage Field, East Texas

Tight gas sandstones of the Cotton Valley reservoirs in Carthage field possess average porosities of <8% and permeabilities <0.1 md. Reservoir quality, however, varies significantly throughout the 427 m stratigraphic interval and, as a result, there is only 30-60 m of net porous sandstone. Assessment of both sequence stratigraphic and depositional controls on reservoir quality is based on correlation of 300 wireline logs, facies analysis of 823 m of core and petrographic examination of 300 thin sections from 10 subsurface cores.

The Cotton Valley section, from 2530-2955 m, consists of six third-order sequences which can be divided into highstand, transgressive and lowstand system tracts. Most of the section is composed of highstand deposits that consist of aggrading to prograding wave dominated shoreline parasequence sets. Early precipitation of chlorite and illite in sandstones deposited in inlet channel and backshore environments preserved primary intergranular porosity by inhibiting compaction and quartz cementation. These sandstones have the highest reservoir quality possessing 15 to 20% porosity and up to 100 md permeability. Sandstones deposited in the high-energy upper-shoreface environment and those deposits containing abundant detrital shell material were cemented either by early calcite or quartz ce ent and have the poorest reservoir quality with porosities <4%. Lower-shoreface sandstones are finer grained, more poorly sorted and enriched in feldspar and lithic fragments in comparison to upper shoreface sandstones. Cementation was not as extensive thereby preserving some intergranular porosity. Moreover, dissolution of feldspars and lithic fragments enhances primary porosity so that total porosity ranges from 6 to 10%. Lower-shoreface deposits are volumetrically the most significant reservoirs at Carthage field.

Distal lower-shoreface deposits dominate the transgressive system tracts and are of very poor reservoir quality due to the extremely fine grain size, high detrital clay content and extensive cementation.

Fluvial-deltaic/estuarine sandstones deposited within incised valleys constitute most of the lowstand system tracts. Although spacially and temporally restricted these thick (30 m), relatively coarse sands are some ofthe best reservoirs displaying average porosities of 12 to 20%.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.