Controls on Reservoir Quality of the Carbonates of the Wolfcamp A in the Eastern Midland Basin (Howard County, Texas): A Case for Active, Open-System Diagenesis

Abstract

The Wolfcamp A toe-of-slope, eastern Midland Basin comprises unconventional-reservoir mudstones and carbonates with pore character of conventional reservoirs. In such hybrids, porosity in the carbonate-rich units can contribute greatly to production. Few diagenetic studies exist for such hybrid deposits. To address this, we tested the hypothesis that fine-grained, low permeability strata created a relatively closed diagenetic system that limited alteration, prevented fluid flow, and was predictable based on closed-system rules of diagenetic alteration. If that hypothesis proves to be incorrect, then the project also evaluates what other diagenetic processes control porosity. Methods are: paragenesis using transmitted light, UV, and CL petrography; SEM-EDS; fluid inclusions: δ¹³C; δ¹⁸O; and ⁸⁷Sr/⁸⁶Sr.

The paragenesis shows: early bioclast dissolution, mechanical and chemical compaction, pyrite, anhydrite, calcite cements, dolomite cements, quartz cements, vein opening, fracturing, late dissolution, and petroleum migration. Fluid inclusion results (mean T_h°C, mean salinity wt% NaCl equiv.) are: anhydrite (n/a, 21.5%); calcite 2 (70 °C, 17.2%); calcite 4 (78 °C, 23.1%); calcite 5 (145 °C, 23.5%); and quartz 2 (86 °C, 24.7%). Calcite δ¹⁸O and δ¹³C show depletion from calcite 2-4, and then enrichment from calcite 6-8. These data indicate three stages of fluid evolution: (1) temperature increase; (2) introduction of an especially hot fluid; and (3) temperature decrease. This complex paragenesis falsifies the closed-system hypothesis.

Controls on porosity include primary macroporosity (interparticle and intraparticle). Mud-poor fusulinid facies ended up completely occluded by cements. Conversely, mud-rich fusulinid facies preserved intraparticle porosity with less cementation. We hypothesize this phenomenon is related to initial permeability, where grainy facies allow greater fluid flow and cementation. Extant secondary porosity includes vein, fracture, dissolved bryozoans, and intercrystalline. The control on this secondary porosity is still unknown, but it may be related to fracture density and late fluid flow.

This study advances the diagenetic model for hybrid systems and sheds light on how open the diagenetic system is and what impact this has on porosity. It can be applied in this region as well as in areas with similar geologic histories where carbonate sediment gravity flows interfinger with basinal mudrock.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018