Diagenetic controls on porosity, thermal history, and hydrocarbons in the Wolfcamp A, eastern Midland Basin, Texas

Abstract

A detailed study of a Wolfcamp A core from the eastern Midland Basin, west Texas, is used to understand the diagenetic history of the region and the controls on carbonate macroporosity. Transmitted-light microscopy, UV-epifluorescence, cathodoluminescence, and SEM-EDS microscopy, as well as δ18O, δ13C, 87Sr/86Sr, and fluid inclusion analyses were performed. The diagenetic history could be divided into stages: Early Stage occurred near the sediment-water interface; Burial Stage occurred during burial and before tectonic fracturing; and Fracturing Stage occurred with the onset of tectonic fracturing and overlaps with late events of hydrocarbon migration. Macropores in carbonate mud-poor packstones were largely occluded by compaction, Early Stage and Burial Stage calcite cements, and host little extant porosity. Mud-rich packstones, however, host both primary intraparticle and late moldic porosity and are better reservoir facies. Within the Burial Stage (Leonardian or Guadalupian) highly saline evaporated seawater (17.4 – 18.5 wt%) refluxed into the Wolfcamp A, displaced connate water, and was heated to burial temperatures (68 – 83 °C) before precipitating calcite cement. 87Sr/86Sr of the calcite cement is more radiogenic than Leonardian or Guadalupian seawater, which is likely due to rock-water interaction with more radiogenic siliciclastics. Afterwards, in the Ochoan, even more highly saline evaporated seawater (17.4 – 26.8 wt%) refluxed and mixed with or displaced the Leonardian or Guadalupian refluxed fluid; it precipitated anhydrite cement during active reflux, prior to being heated to burial temperature. 87Sr/86Sr of the anhydrite cement (0.707491) supports precipitation from Ochoan seawater. At the onset of the Fracturing Stage, there is a fracturing event and a dramatic rise in fluid temperatures from 68 – 93 °C to 130 - 160 °C, which is well above the modeled maximum burial temperature of 85 °C. Mean δ18Ocement shifts to markedly more negative values from the Burial Stage (-2.77‰) to the Fracturing Stage (-5.28‰), which supports an increase in temperature. Characteristics of the Th record indicate tectonic valving of hydrothermal fluids. Relatively non-radiogenic 87Sr/86Sr for high temperature cements (0.707464 – 0.707846) suggest a fluid origin from a sedimentary basin rather than basement. The most likely source for the hydrothermal fluids is the much deeper Val Verde Basin. A modern geothermal gradient map shows a high-gradient plume, directed northward from the Val Verde Basin, with NE-SW- and NW-SE-oriented boundaries. NE-directed Laramide compressive stress resulted in NE-SW and NW-SE fracture sets. Laramide fracturing in the Val Verde Basin and Midland Basin resulted in long-distance (at least 300 km) northward migration of deep hot fluids along these NE-SW and NW-SE fracture zones within certain stratigraphic intervals. Cenozoic uplift south and west of the Val Verde Basin resulted in meteoric water infiltration into deep reservoirs. This caused northward fluid displacement and migration of hydrothermal fluids. The paragenesis indicates that much hydrocarbon generation and migration occurred in the Cenozoic, which is much more recent than previously estimated.