--> Abstract: Diagenesis in the Delaware Mountain Group--An Example of Organic-Inorganic Interaction during Burial, by P. Hays; #91012 (1992).

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ABSTRACT: Diagenesis in the Delaware Mountain Group--An Example of Organic-Inorganic Interaction during Burial

HAYS, PHILIP, Texas A&M University, College Station, TX

The Guadalupian Delaware Mountain Group, including in descending order the Bell Canyon, Cherry Canyon, and Brushy Canyon formations, is composed of very fine grained, subarkosic sandstones and siltstones. Cored intervals from depths of 600 to 2500 m, from Reeves and Pecos Counties, Texas, and Eddy County, New Mexico, show that reservoir sandstones are interbedded with thin limestones and thick, organic-rich siltstones that comprise up to 80% of the sequence. Shales and detrital clays are notably rare.

Delaware Mountain Group siltstones have an average total organic carbon content of up to 11%, averaging about 3%. Rock-Eval pyrolysis data show that siltstone organic matter is oxygen-rich type II and II kerogen, prone to generation of significant quantities of carboxylic acids and CO2 during thermal maturation. Rock-Eval Tmax data and various biomarker maturity indicators show that the organic matter is at a moderate stage of maturity, well within the oil generative phase. Correlation of carbon isotopic compositions, trace metal content, and specific elemental ratios between organic and inorganic phases provide evidence that diagenetic processes in the sandstones were driven by organic degradation. In addition, Delaware Mountain Group siltstone organic matter appears to have been the source for much of the oil reservoired in Delaware Mountain Group sandstones. Carbon and sulfur isotopic data and biomarker data show good correlation between Delaware Mountain Group source rock and Delaware Mountain Group oils produced in Pecos and Reeves Counties, Texas, and Eddy County, New Mexico. However, Brushy Canyon oils from Loving County, Texas, appear to belong to a different oil family. Biomarker content indicates that Delaware Mountain Group sourced oils are derived primarily from the type II kerogen, containing only traces of compounds peculiar to Delaware Mountain Group type III kerogen.

Petrologic study shows that the reservoir sandstones have experienced four major episodes of diagenetic alteration: (1) early cementation by carbonate, sulfate, and halite cements that partially arrested compaction and preserved significant intergranular volume during

progressive burial; (2) extensive dissolution of cements and some detrital material to form voluminous secondary porosity; (3) extensive chlorite authigenesis that dissected nearly half of the porosity into microporosity; (4) authigenesis of dolomite, feldspar, and Ti-oxides during the late period of diagenesis. Characterization of the organic geochemistry of Delaware Mountain Group siltstones and analysis of late authigenic products in the sandstones indicate that organic alteration in surrounding organic-rich siltstones controlled pore fluid chemical evolution within the sandstones from early to late burial. Chemical conditions maintained within the reservoir sandstones resulted in creation of secondary porosity and the suite of authigenic minerals observed.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)