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ABSTRACT: Organic Matter in Delaware Mountain Group Sediments -- Geochemistry, Source, and Role in Diagenetic Alteration of Reservoir Sandstones

HAYS, PHILLIP D., and THOMAS T. TIEH, 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.

Rock-Eval pyrolysis data show that Delaware Mountain Group siltstone organic matter is oxygen-rich type II and type III 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, Ni, V, U, Th, Fe, and Mn element content, and specific elemental ratios between organic and inorganic phases indicate 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 sand tones. 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 partly 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, which dissected nearly half of the porosity into microporosity; (4) authigenesis of dolomite, feldspar, and Ti-oxides during late diagenesis. Characterization of the organic geochemistry of Delaware Mountain Group siltstones and analysis of late authigenic products in the sandstones indicates that organic diagenesis controlled pore fluid chemical evolution within the sandstones and resulted in creation of secondary porosity there. Authigenic minerals in the sandstones exhibit isotopic and trace element content indicative of this organic influence.


AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)