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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 9. (September)

First Page: 1430

Last Page: 1431

Title: Effects of Diagenesis on Reservoir Properties and Log Response, Upper Jurassic Taylor Sandstone, Cotton Valley Group: ABSTRACT

Author(s): Michael Trojan

Article Type: Meeting abstract

Abstract:

The Taylor Sandstone in Terryville field, Lincoln Parish, Louisiana, is one of many tight gas-bearing sandstones of the Upper Jurassic Schuler Formation, Cotton Valley Group. This coastal strand-plain sandstone is fine to very fine grained, well sorted, with grains that are subrounded to well rounded. It is highly quartzose with an abundance of carbonate cement and lesser concentrations of quartz cement, clay precipitates, and pyrite.

Mechanical compaction has caused a 10% reduction in primary porosity with increasing overburden. Chemical diagenesis has altered the texture and composition of the sandstone and has affected porosity and permeability through cementation, dissolution, and authigenic clay precipitation. Early stages of diagenesis included pyritization, mechanical compaction, and hinderance of compaction by precipitation of quartz cement at grain contacts. The middle stages were dominated by carbonate cementation, which replaced large amounts of detritus and quartz cement and reduced primary porosity to irreducible limits. Finally, the latest stages of diagenesis included development of secondary porosity by localized dissolution of replacive and interstitial carbonate and reduction of porosity by precip tation of pore-lining and pore-filling illite and illite/smectite clays. Poor permeabilities in the sandstone are a direct consequence of incomplete dissolution of carbonate in pore throats and the obstruction of voids by the late clay precipitates.

The conductive property of authigenic pyrite has affected the response of the deep induction resistivity log, consequently causing abnormally high calculated water-saturation values in the Taylor Sandstone. The effect pyrite has on formation resistivity can be clearly seen on Rt-^phgr plots (Pickett method). Formation resistivity is affected even at low percentages of pyrite, and shows an exponential decrease with increasing pyrite concentration. A correction of resistivity was made possible by determining

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pyrite concentrations in the laboratory. Furthermore, the Rt-^phgr plot may confirm the presence of pyrite in a sandstone where core samples are not available.

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Copyright 1997 American Association of Petroleum Geologists