Deep-burial microporosity in upper Jurassic Haynesville oolitic grainstones, East Texas

Abstract

Secondary micromoldic porosity generated during deep-burial diagenesis occurs pervasively in Upper Jurassic Haynesville oolitic grainstones in East Texas and constitutes the major pore type in these gas reservoirs. Petrographic and geochemical relationships establish that development of this microporosity postdates emplacement of bitumen and most pressure solution fabrics in the reservoir grainstones. Microporosity development is strictly controlled by depositional texture and is restricted to either active shoal complex grainstones or thicker grainstones shed downramp by storm processes. Haynesville diagenetic and porosity relationships are consistent along the entire length of the east flank of the East Texas Basin, a distance greater than 100 km; identical relationships have also been observed along the west flank of this basin. Haynesville micromoldic porosity development is confined principally to ooids but also occurs in normally “stable” calcitic skeletal grains like oysters. Resultant micropores are a few microns across or less; complete dissolution of ooids to form oomoldic macroporosity is not observed in Haynesville limestones. Nearly all primary porosity in the Haynesville is now occluded by carbonate cement. Confirmation of a late, deep-burial origin for Haynesville secondary microporosity is based on physical relationships observed in numerous cores, regional petrography and geochemical data. Collectively, these observations demonstrate that Haynesville sediments were never locally or regionally exposed to freshwater but have undergone progressive burial diagenesis punctuated by a major late dissolution event which created the microporosity. Key observations which support Haynesville deep-burial microporosity development include: (1) petrographic relationships which demonstrate microporosity developed after emplacement of bitumen; (2) lack of subaerial exposure features in core, both atop shoal complexes and at the contact between the Haynesville and Bossier sequences; (3) absence of freshwater porosity fabrics; (4) lack of precompactional freshwater cements; (6) pervasive pressure solution in the microporous grainstones, including extensive grain interpenetration; (6) preservation of abundant microporosity directly adjacent to pressure solution seams in porous grainstones; (7) development of identical microporosities in mineralogically stable calcitic grains which rarely leach in freshwater; (8) microporosity development which postdates formation and cementation of fractures in the reservoir facies; and (9) calcite cements occluding primary porosity whose geochemical attributes are inconsistent with precipitation from freshwater but are consistent with a burial origin. This regional, core-based case study reveals that the contact between the Haynesville and overlying Bossier Shale is a marine “drowning unconformity” Physical, petrographic and geochemical evidence for regional subaerial exposure at the end of Haynesville deposition, due to a eustatic fall in sea level, is not present, questioning the validity of interpreting local or global sea level history from seismic and well log data in the absence of critical core control.