Abstract
Paleoredox proxies (Fe speciation, trace element and δ34Spy) integrated with sedimentological and palynological observations link the distribution and type of particulate organic matter (OM) preserved to hydrocarbon source rock potential. In the Mississippian Bowland Shale Formation (Lancashire, UK), particulate OM is dominated by “heterogeneous” amorphous OM (AOM), primarily “sharp-edged, pellet-like” (AOMpel) and “heterogeneous, granular” (AOMgr) types. AOMpel is abundant in muds deposited under anoxic and moderately to highly sulphidic conditions and most likely represents the fecal minipellets of zooplankton and/or pellets of macro-zooplankters. We recognize two intervals, “A” and “B,” which exhibit Sorg/TOC > 0.04, suggesting a bulk Type II-S kerogen composition. The Interval A palynofacies is typified by pyritized AOMpel (AOMpyr) particles that contain high-relief organic spheres surrounding individual pyrite framboids, within each AOMpyr particle. These textures are interpreted as sulphurized OM local to pyrite framboids (Sorg-PF). Sorg-PF is rarely observed in Interval B, and absent in all other samples. Redox oscillation between ferruginous and euxinic conditions during early diagenesis of Interval A likely promoted S cycling in microenvironments surrounding pyrite framboids, which generated reactive S species and reactive OM required for sulphurization. Early diagenetic redox oscillation processes were apparently triggered by relative sea level fall, associated with an increased supply of FeHR from adjacent shelves into the basin. Interval B represents deposition during the late stages of basin infill and transition from anoxic to (sub)oxic bottom waters, where AOMpel is replaced by AOMgr as the dominant type of AOM. A large particle diameter at the limit of the mesh size (500 μm), sheet-like, fragmented character, and presence of candidate organic sheaths suggests AOMgr at least partially represent fragments of benthic microbial mats, probably as sulphide-oxidizers. A ternary plot of AOMpel + AOMpyr versus AOMgr versus spores + phytoclasts links the observed palynofacies to bottom and pore water redox conditions, water column productivity and proximity to fluvial (deltaic) supply of spores and phytoclasts. These variables were moderated by changing basin accommodation, driven primarily by eustatic sea level fluctuation. A sequence-stratigraphic control on AOM type and sulphurization is important for understanding the link between source rock heterogeneity and the timing of hydrocarbon generation and expulsion from this source rock.
Highlights
The upper unit of the Mississippian Bowland Shale Formation (Upper Bowland Shale; “Bowland Shale”) is a potential target for unconventional hydrocarbon exploration and an important conventional hydrocarbon source rock in the UK
Plotting heterogeneous amorphous OM (AOM) versus phytoclasts versus spores (Fig. 4) demonstrates most Facies A–G samples are sited in the “distal suboxic–anoxic basin” and “distal dysoxic–anoxic shelf” fields of Tyson (1995)
Anoxic conditions in bottom waters during deposition of Facies A–F sediments are supported by enrichment in redox-sensitive trace elements Mo and U (e.g., Tribovillard et al, 2006) and FeHR/FeT N 0.38 (Figs. 2, 4–5; Table 1) (Poulton and Raiswell, 2002; Raiswell and Canfield, 1998)
Summary
The upper unit of the Mississippian Bowland Shale Formation (Upper Bowland Shale; “Bowland Shale”) is a potential target for unconventional hydrocarbon exploration and an important conventional hydrocarbon source rock in the UK In Mississippian organic-rich mudstones, in particular, the extracted palynological OM fraction (e.g., Wood et al, 1996) is dominated by amorphous OM (AOM; e.g., Könitzer et al, 2016; Hennissen et al, 2017) This is especially typical of Mississippian source rocks, given the lack of recognizable marine palynomorphs during the “Phytoplankton Blackout” (Pitrat, 1970; Riegel, 1996; Riegel, 2008; Servais et al, 2016; Tappan, 1970). These productive (low stress) conditions potentially supported phytoplankton which did not produce cysts, considered marine palynomorphs with relatively high preservation potential, because encystment was unnecessary (Servais et al, 2016)
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