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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 988

Last Page: 988

Title: Core and Log Analysis for Subtle Trap: Cotton Valley Tight Gas Sand: ABSTRACT

Author(s): Joseph A. Schrank, Ercill Hunt

Article Type: Meeting abstract


Several Cotton Valley zones that appear wet from standard log calculations have proven to be gas productive in recent months. In an attempt to understand this phenomenon, a case study was performed in conjunction with Champlin Petroleum Co. on cores from the Carthage field, east Texas. The zone studied was of low resistivity (less than 2^Ogrm) which, when combined with porosity (16%), calculates very high water saturation (> 60%) using the standard Archie equation and m = n = 2.

Geologic studies indicate the low resistivity zone was deposited as a middle to upper bar sequence exhibiting massive and laminated bedding while a normal resistivity zone (~ 8^Ogrm) immediately below exhibits both a dense characterless section and a lower bioturbated facies with marine trace fossils.

Examination of the pore system using an SEM shows authigenic illite (identified by XRD) lining the pores and pore throats in both zones. Authigenic clay precipitation was subsequent to the development of quartz overgrowths and prevented complete occlusion of the pore system by the quartz. Secondary intergranular porosity is developed where feldspar grains have undergone dissolution.

The total porosity of the low resistivity interval (16%) is twice that of the normal resistivity zone (8%). Permeability is low throughout the interval (< 20 md), but somewhat higher in the low resistivity zone. Cation exchange and surface area measurements indicate that the authigenic clay is of equal volume in each zone. However, electrical measurements suggest greater rock conductivity and lower m and n Archie parameters in the low resistivity zone. The petrophysical and geologic data collected on the Cotton Valley provided more realistic petrophysical parameters for water saturation calculations, and led to a better understanding of the pore network and origin of the low resistivity interval.

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Copyright 1997 American Association of Petroleum Geologists