LITHOFACIES, DIAGENESIS, AND RESERVOIR QUALITY OF THE UNCONVENTIONAL WOLFCAMPIAN SUCCESSION IN THE SOUTHEAST MIDLAND BASIN, WEST TEXAS

The Lower Permian Wolfcampian basinal succession in the Midland Basin has become an important unconventional reservoir. This study combined SEM (scanning electron microscope) results, thin section observations, X-ray fluorescence data, X-ray diffraction data, and total organic carbon data in order to provide an integrated characterization of the Wolfcamp succession.

Lithofacies investigations were made using six cores (totally 667 ft) from the southeast Midland Basin. Seven lithofacies were defined based on petrographic observations: (1) sandy siltstone, (2) argillaceous mudstone, (3) very fine to fine sandstone, (4) massive to weakly laminated calcareous mudstone, (5) laminated calcareous mudstone, (6) mud-dominated bioclastic packstone/rudstone, and (7) grain-dominated bioclastic packstone-grainstone/rudstone. Depositional processes include hemipelagic settling, diluted turbidity flows, turbidity flows, debris flows, and hyperconcentrated density flows deposits. These sediments were deposited in a deep-water, dysoxic to anoxic slope to basinal setting. High-frequency cyclicity is observed in meter scale, occuring as a relative carbonate-rich lithofacies overlain by a relative siliciclastic-rich lithofaices.

The Wolfcamp succession reveals a complex diagenetic history in sandstone and carbonate facies, including compaction, calcite and silica cementation, siderite coating and dissolution. The combined effects of compaction and cementation result in relatively low porosity. However, dissolution and siderite grain-coating in the sandstone create/save pore spaces for hydrocarbon storage. Measured core-plug porosity and permeability in sandstone succession suggest moderate porosity up to 11.6%. Siderite-coated sandstone is considered to be of the highest reservoir potential based on the results. The Wolfcamp contains fair to good organic matters with an average TOC of 1.4%. The enrichment of organic matter increases with increasing primary productivity by showing positive relation with Ni, Cu, and P element. Organic enrichment also increases with bottom water anoxia by showing positive relation with Mo and U elements. TOC enrichment is also affected by sediment influx during the early Wolfcampian.