Depositional systems interpretation of early Permian mixed siliciclastics and carbonates, Midland Basin, Texas.

摘要

In this study, a core from the Midland basin of West Texas was examined in detail to determine the depositional setting of an Upper Wolfcampian and Lower Leonardian-aged mudstone succession. The studied sediments were deposited close to the center of the Midland Basin in a deep water, mixed carbonate and siliciclastic system. Core X was described on a centimeter scale and lithofacies were defined on the basis of mineralogy, texture, sedimentary structure, and degree of bioturbation. Their main features are summarized from bottom to top of the cored interval. The Wolfcamp B2 and B1 intervals consist of multiple facies types, but are dominated by organic rich silty mudstone, skeletal argillaceous packstone, and bioclast-lithoclast floatstone. The Wolfcamp A3 and A2 intervals are dominated by fine grained, resedimented carbonates in the form of grainstone beds that grade upward into packstone and mudstone. Structures attributed to post depositional slumping are common. The Wolfcamp A1 interval consists of thin grainstone beds and thick sections of interbedded silty mudstone and muddy siltstone. The Dean Formation overlies the Wolfcamp interval and consists of thick beds of very fine grained, well cemented, sandstone and muddy siltstone. Of the described facies, the clay and silt rich varieties are interpreted as hemipelagic sediments deposited by suspension settling, bottom currents, or dilute density flows. In contrast, carbonate rich facies are interpreted as deposited by a spectrum of sediment gravity flow processes. Moderate to high levels of bioturbation in all clay and silt rich facies suggests dysoxic to suboxic conditions within the basin during the time of deposition. An SEM investigation of pore types and their distribution led to the recognition of three pore types: phyllosilicate framework pores, organic matter pores, and dissolution pores. SEM observations and conventional core analysis indicate that the greatest porosity occurs in the clay and silt rich facies, whereas grainstones and cemented sandstones are the least porous. Rock Eval pyrolysis data were integrated with the core description to help identify the most organic rich facies. Clay and silt rich facies show the highest average TOC, whereas the grainstone facies show the lowest average TOC values.