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Carbonate-Dominated Hybrid Sediment Gravity Flows Within the Upper Wolfcamp, Delaware Basin, USA: Vectors for Transmitting Terrestrial Organics Into a Deep Marine Basin

Abstract

The Delaware Permian Basin in southeast New Mexico and west Texas is currently one of the hottest areas for oil and gas exploration in North America. Regional stratigraphic studies within the lower Permian Wolfcamp interval reveal that basin sediments were derived from both siliciclastic and carbonate clastic sources dispersed around the basin. Carbonate deposition within the basin was dominated by sediment gravity flows including mass transport complexes (that can include debris flows, rafted blocks of preexisting slope deposits, and shelf material) and hybrid event beds (HEBs-also known as linked debrites). Carbonate HEBs are common along the western flank of the Central Basin Platform and out into the basin. Single flow events up to 10 meters thick have been recognized in cores obtained from basin floor settings, but most are much thinner. The HEBs are composed of diagenetically altered fusulinid-rich skeletal packstones. Often the packstones are massive but some also exhibit laminated bedding. Silt-size quartz grains are also a component. In a complete succession, the packstones (Ps) will grade upwards to a mostly silty packstone (SLc) that may be weakly laminated. This is then typically sharply overlain by a calcareous argillaceous siltstone (SLca) that is massive but often shows a “clotted” texture that may represent fluid escape structures that were sheared during final deceleration of the deposit. The SLca grades upward into a calcareous silty mudstone (Mcs) that is also massive. A slightly calcareous to non-calcareous massive to laminated mudstone (Mm/Ml) sharply to gradationally overlies the calcareous mudstone. The massive calcareous muddier portions of the HEBs (SLca, Mcs) exhibit TOC values that can range from 0.6% to 3.5%, whereas the carbonate-dominated basal portions (Ps, SLc) generally contain less than 1%. The non-calcareous mudstones (Mm/Ml) that cap the event beds contain as much as 8% TOC. Within the condensate window, the more organic rich uppermost parts of the HEBs constitute the reservoir facies. A significant proportion of the TOC within the calcareous mudstones and non-calcareous mudstones may have been derived from terrestrial organics as evidenced by well-preserved fern-like plants with fully articulated leaves (Germaropteris martinsii). Their presence within the HEB indicates that the Central Basin Platform was subaerially exposed during their deposition with plants growing on shelf-edge islands.