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

Abstract

Aqueous species derived from first-order degradative reaction of organic matter can modify Eh and pH conditions in the burial environment and regulate solubility characteristics of other aqueous species and minerals in the rock-water system. While this potential of organic material in sediments to control pore water chemistry and diagenetic processes has been well demonstrated theoretically and experimentally, data on actual field examples of coupled organic-inorganic diagenetic systems are limited. Several studies have promoted organic matter alteration as an important diagenetic process, especially for creation of secondary porosity. However, these studies are hotly debated by workers who believe that it is difficult or impossible to resolve the individual diagenetic effects of the various fluid sources driving diagenesis, e.g. meteoric water, water derived from clay transformation, or water derived from degradation of organic material. This study evaluates the role of organic matter in controlling diagenetic processes in 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 elastic-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. To fully understand the evolution of pore fluid chemistry in the Delaware Mountain Group, detrital and authigenic mineralogy, diagenetic history, and organic geochemistry of the rocks and fluids were characterized in detail...