An integrated diagenetic and paleomagnetic study of the Haynesville Shale, Harrison County, Texas

Abstract

An integrated diagenetic and paleomagnetic study of the Upper Jurassic Haynesville Shale was undertaken to determine the origin and timing of diagenetic events in this prolific shale-gas unit. Characterization of diagenesis in the Haynesville has not previously been conducted utilizing paleomagnetism. Distinguishing different diagenetic events and their extent within the reservoir has scalable impacts on production within the Haynesville Shale, particularly as related to the presence of natural vertical healed fractures. Two unoriented cores from Harrison County, Texas were sampled, described, and utilized in this study.

Five facies were described through petrographic analysis: siliceous mudstone, laminated siliceous mudstone, carbonate-rich mudstone, burrowed mudstone, and dolostone. Scanning electron and transmitting microscope analysis shows a complex paragenetic sequence within these facies and associated healed fractures including the presence of sphalerite, barite, celestite, anhydrite, double terminating quartz, and leucoxene. Horizontal veins are interpreted as early diagenetic events, and vertical veins and brecciation are interpreted as late events. Heterogeneity within vertical veins may indicate multiple activation events and certainly multiple exposures to diagenetic fluids.

Through thermal demagnetization a low temperature component was uncovered with moderate to steep inclinations interpreted as a viscous remanent magnetization (VRM) in a few specimens. An intermediate temperature component (160-320_) with moderate to steep inclinations is interpreted as a chemical remanent magnetization (CRM) residing in pyrrhotite. A higher temperature component (300-480 _ with moderate inclination is also interpreted as a CRM, with magnetization held in magnetite. Low temperature demagnetization resulted in a small decrease in magnetic intensity and in some cases increased maximum unblocking temperatures of the intermediate and higher temperature components. A drilling induced remanent magnetization (DIRM) was ruled out through Q-factor analysis. Inclination values of intermediate and high temperature CRMs from both cores are utilized because few specimens had a well-defined VRM that could be used for core orientation. Inclinations for the CRM residing in magnetite show possible remagnetizations occurring in the Late Jurassic/Early Cretaceous or Late Cretaceous/Middle Cenozoic. The intermediate temperature CRM is a Late Cretaceous to Cenozoic remagnetization. Because the intermediate temperature CRM may have an inclination direction associated with the Cenozoic, core orientation of the Snider A 1-H was attempted through rotating these directions to the present-day direction (0_ Based on the rotation, the magnetite CRM has a good grouping and a Paleogene/Neogene pole position on the apparent polar wander path (APWP). The two cores contain moderate total organic carbon (TOC) values (~2-4 wt%) and the origin of magnetite remanence may be attributed to burial processes such as maturation of organic matter which occurred at ~140 Ma. Other burial processes such as dissolution-reprecipitation of pyrite may have triggered precipitation of authigenic magnetite. Alternatively, late diagenetic basinal fluids could have caused a Paleogene/Neogene remagnetization. A presence/absence test around veins in the cores yielded no conclusive results in regards to basinal remagnetizing fluids. The pyrrhotite remanence is interpreted as forming through thermochemical sulfate reduction (TSR) relating to hydrocarbons perhaps as early as the Late Cretaceous.