Fluid evolution in fracturing black shales, Appalachian Basin

Abstract

Opening-mode veins in cores drilled from the mudrocks over-and underlying the major Silurian salt décollement in the Appalachian plateau (Tioga and Lawrence Counties, Pennsylvania), have mineralogic and isotopic compositions generally matching those of their host mudrocks, suggesting opening and filling amid little cross-stratal fluid motion. Calcite and most trace minerals probably entered the veins via dissolution-reprecipitation from nearby host rock. Consistent with this interpretation are the observations that (i) trace minerals within the veins, including quartz, pyrite, and dolomite, are invariably also present within the layers hosting the veins, with vein cement minerals generally reflecting the abundance and solubility of minerals in the host-rock, and (ii) carbon and oxygen isotopic compositions of vein-filling calcite are similar to those of calcite within the host rock, with vein-filling δ 18 O slightly depleted and δ 13 C slightly enriched. Modeling the fluid isotopic evolution, assuming vein opening and filling amid immobile connate formation water, accounts for these minor but systematic differences, which are attributable to increasing temperature and hydrocarbon maturation. An exception to the above trend is barite, which, despite its low solubility, is systematically enriched in veins with respect to the host rock. It is unclear whether barite precipitation resulted from the influx of external fluids-perhaps deriving from Silurian salt-or from barium mobilized at depth from local clays or organic material.

Shale Anisotropy and Natural Hydraulic Fracture Propagation: An Example from the Jurassic (Toarcian) Posidonienschiefer, Germany

“…If the expulsion of the petroleum products had been hindered by the generally low permeability of the host source‐rock, it is possible that the intensity of the hydraulic fracturing driven by the maturation process would also have increased (see Mandl & Harkness, ). A low‐permeability, generally closed fluid system is also consistent with previous laboratory studies of shales (Neuzil, ), and with the vein fills comprising minerals that are also abundant within the host rock (Hooker, Cartwright, et al, ). Our observations therefore support those of Jochum et al () that layer‐parallel veins were formed by volume increase and overpressure related to the maturation of organic matter; i.e., the veins are natural hydrofractures driven by catagenesis.…”

Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine sequence stratigraphic framework, Dongying Depression, Bohai Bay Basin, Eastern China

“…This is in agreement with the oil window (2700 m) of the Sq3-2 to Sq3-1 and Sq2-2 source rocks. This evidence shows that overpressure in the depression was likely caused by hydrocarbon generation, if the rate of volume increase resulted from the transformation of high-density organic matter to low-density fluids (oil or gas) exceeded the rate of volume loss caused by fluid migration and expulsion (Berg et al 1999;Lee and Williams 2000;Fossen et al 2010;Dasgupta et al 2016;Hooker et al 2017). This overpressure mechanism relies on the kerogen type, density of organic matter, rock permeability, and thermal history (Osborne and Swarbrick 1997;Tingay et al 2009;Dixit et al 2017).…”

“…In general, mechanism for overpressure includes compaction disequilibrium (Rubey and Hubert 1959;Magara 1975;Mara et al 2009;Ramdhan and Goulty 2011;Lahann 2017), hydrocarbon generation (Bredehoeft et al 1994;Guo et al 2010), and fluid release during dehydration reactions (Magara 1975;Hooker et al 2017). However, clay dehydration cannot generate significant overpressure unless there is a perfect seal (Luo and Vasseur 1992;Osborne and Swarbrick 1997).…”

“…Some researchers think compaction disequilibrium is the dominant mechanism for overpressure generation in the Dongying Depression (Xie et al 1998;Xie 2001). The reason is if fluids cannot be expelled sufficiently when a formation is increasing rapidly, the disequilibrium compaction occurs rapidly (Osborne and Swarbrick 1997;Hooker et al 2017). However, in Dongyin Depression, there are no apparent characteristics of compaction disequilibrium, such as anomalously high porosities or low density in the overpressured mudstones (Guo et al 2010).…”

Influence of nodular structures on fracture development in fine-grained rocks: numerical simulations based on the discrete element method

“…the Barnett Shale in the Fort Worth Basin (USA) (Bowker, 2007), the Marcellus Shale in the Appalachain Basin (USA) (e.g. Hooker et al, 2017), the Vaca Muerta Shale in the Neuquén Basin (Argentina) (e.g. Rodrigues et al, 2009), the Low Lias Shale in the Wessex Basin (UK) (e.g.…”