The role of organic matter alteration in sediment diagenesis: The Delaware Mountain Group of west Texas and southeast New Mexico

The Delaware Mountain Group lies within the Delaware basin and includes, in descending order, the Bell Canyon, Cherry Canyon, and Brushy Canyon Formations. This clastic-dominated basinal sequence is composed primarily of very fine-grained, subarkosic sandstones and siltstones. Thin intervals of permeable sandstone are interbedded with thick, organic-rich siltstones that comprise up to 80% of the sequence. Shales and detrital clays are notably rare. This sequence of rocks presents a unique opportunity to study coupled organic-inorganic diagenesis for several reasons: (1) the organic-rich nature of the siltstones provides a voluminous organic material source, (2) the lack of shale precludes control of pore water chemistry by clay transformation, (3) the range of meteoric influx into the basin is known and is limited to the basin margins, and (4) the thick, uniform character of the sequence minimizes lithologically-controlled variables. Natural tracers establish a source-sink relationship between siltstone organic matter and sandstone authigenic phases and illucidate the influence of organic matter in sandstone diagenetic processes.; Petrologic study shows that Delaware Mountain Group sandstones have experienced four major episodes of diagenetic alteration: (1) early cementation by calcite cement, (2) dissolution of calcite and some detrital material to form considerable secondary porosity; (3) chlorite authigenesis, and (4) authigenesis of dolomite, feldspar, and Ti-oxides during the late stages 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. During early burial, degradation of organic matter through sulfate reduction provided alkalinity and calcium ions necessary for precipitation of calcite. During later burial, thermal alteration of organic matter yielded carboxylic and carbonic acids maintaining chemical conditions within the proximal sandstones that resulted in creation of secondary porosity and the suite of authigenic minerals observed. Stable isotopic and minor element tracers, biochemical marker compounds, and solubility considerations of late stage authigenic products provide evidence of the link between organic alteration in the siltstones and diagenetic processes in the sandstones. (Abstract shortened with permission of author.)...