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Unconventional Reservoir Potential of Rift-Lake Mudrocks: Preliminary Results from the Newark, Deep River, and Central Lake Malawi Basins

Abstract

Once considered purely petroleum source rocks, fine-grained sedimentary rocks are increasingly recognized for their ability to store abundant natural gas. Advances in horizontal drilling and hydraulic fracturing have prompted major shale gas projects throughout North America. However, the vast majority of shale gas produced today is from sediments deposited in complex Paleozoic and Mesozoic marine foreland settings. A new frontier for unconventional gas accumulations may be found in lacustrine basins, whose thick packages of organic-rich strata exist in a variety of tectonic settings and span the geologic record. Herein, we evaluate reservoir potential in three tropical lacustrine rift basins: (1) the Newark Basin (Triassic Lockatong Formation; New Jersey, USA); (2) the Deep River Basin (Triassic Cumnock Formation; North Carolina, USA); and (3) the Central Lake Malawi Basin (Quaternary; Malawi, East Africa). The prospective units in these basins exhibit differing thermal maturity, with immature Lake Malawi Basin mudrocks (%VRo > 0.54) serving as a Quaternary analog for higher-maturity Triassic mudrocks in the Newark and Deep River Basins (%VRo > 2.16 and 1.77, respectively). Suites of well-preserved drillcore samples from each basin were analyzed for organic geochemistry, mineralogy, microfabric, porosity, and gas sorption characteristics. Thick cyclic packages of upward shoaling parasequences were distinctly expressed in the drillcore strata from the Newark and Deep River Basins, with total organic carbon concentrations (% TOC) and Rock-Eval (RE) pyrolysis data indicating gas-prone source potential in a variety of laminated or burrowed microfacies. Feldspar- and carbonate-dominated mudrocks from the Lockatong Formation generally exhibit lower mercury-filled porosity (< 2.8 %) and methane sorption capacity at 65°C than the clay-dominated mudrocks of the Cumnock Formation (< 6.4%). Cyclic packages of upward shoaling parasequences were indistinctly expressed in the Malawi Basin core, with % TOC and RE datasets indicating source rock potential from Type I kerogen in laminated microfacies. Mercury-filled porosity and methane sorption capacity at 65°C in clay-rich Malawi Basin mudrocks achieve the highest values in our sample set. Preliminary results from this study add to a growing unconventional reservoir characterization database of lacustrine deposits, and will be used to further our understanding of gas storage in fine-grained rocks.