Sedimentology, petrography, and diagenesis of selected Paleozoic source rocks from the Permian Basin

Sedimentological and paleontological investigations of the Upper Devonian Woodford, Pennsylvanian (Missourian), and Permian (Wolfcampian) shales from the Permian Basin of west Texas and southeastern New Mexico indicate that these shales were deposited in deep-water environments, whereas the Atokan shales were deposited in a shallow-water open shelf environment. Woodford, and Wolfcampian shales were deposited as hemipelagic sediments in environments having little wave and current action, while Canyon shales were deposited by turbidity currents and are associated with submarine fan facies. Integrated sedimentological, paleontological and geochemical data indicate that these Paleozoic shale sequences were accumulated in stratified water columns and anoxic to near-anoxic conditions prevailed during their deposition. Hence, large amounts of marine organic matter are preserved in these shales, as indicated by the relatively high total organic content (TOC) values. TOC for all shales exceeds 5 wt% and reaches up to 26 wt%.

Shales are classified as, lutite, granolutite, granopackite, and compackite. Lutite is a clay-supported rock with less than 10% grains, whereas granolutite is a claysupported rock with more than 10% grains. Granopackite is a grain-supported rock, having a texture in which grains are in contact and detrital clay is dispersed among the grains. Compackite is characterized by a grain-supported fabric, but the grains are separated by clay platelets which have been bent and distorted owing to compaction. Canyon turbiditic shales exhibit much more shale rock type diversity than any of the Woodford, Atokan, and Wolfcampian hemipelagites.

Appreciable amounts of diagenetic minerals were precipitated in these shales. Framboidal and cubic pyrite and silica are common minerals in all shale sequences. Dolomite, calcite, ferroan dolomite and ferroan calcite are abundant in Wolfcampian shales and significant amounts of ferroan dolomite occur in Woodford silty black shales. Celestite and anhydrite were detected only in the Wolfcampian shales from the Delaware Basin and siderite is only found in the Canyon shales. The significance of abundant authigenic minerals in these shales, is that they occur early, possibly before hydrocarbon generation, hence producing a very impermeable medium which would obstruct migrating pore fluids and/or oil. Clay-mineral diagenesis (conversion of illite smectite mixed layers to illite) appears to partially have influenced the diagenesis or precipitation of the authigenic nonclay minerals in these shales, and played an important role in the evolution of ferroan calcite and ferroan dolomite in Woodford and Wolfcampian shales.

Petrographic analyses of the studied (Woodford, Atokan, Canyon, and Wolfcampian) shales, in conjunction with examination of geophysical density and porosity logs (FDC and CNL) indicate that some shales are compacted, while others are less compacted. Compacted shales are characterized by low densities, contain flattened palynomorphs and shale fabrics compressed around sand grains, bioclasts, and pyrite and phosphatic nodules. Less compacted shales (Wolfcampian shales) contain appreciable amounts of diagenetic minerals which resulted in differential cementation of these shales, increasing their density and arresting compaction.