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Application of Sequence-Stratigraphic Models to Organic-Rich Rocks of the Middle East

Abstract

We developed sequence-stratigraphic models of organic-rich rocks (ORR) since the recognition of the condensed interval in the 1970's. These models aid in the prediction of the occurrence, geometry, and composition of ORR for exploration of conventional oil and gas and unconventional shale oil, shale gas, and oil shale. Marine ORR are most common in the Middle East and are explained by three basic models: constructional shelf margin, platform/ramp, and continental-margin. The constructional shelf margin is the most common setting for ORR. It is the classic condensed interval with maximum organic enrichment occurring at the mid-sequence downlap surface (maximum flooding surface or MFS). Thickness of the ORR typically decreases basinward, whereas TOC values increases as the distance from diluting detrital sediments increases. HI values similarly increase basinward, as land-plant material becomes less common. In vertical profile, TOC and HI values increase up to the MFS and then decrease in the overlying highstand strata. Middle East examples occur in the Pabdeh, Kazhdumi, and Qusaiba formations. The Lam and Meem formations are constructional shelf margin ORR with extremely high terriginous sedimentation rates. Carbonate shelf margins with minimal terriginous deposition are common in the Middle East. Basinal carbonate ORR deposited in outer-neritic to upper-bathyal water depths tend to be thinner with high TOC values. HI values are high throughout the deposit, reflecting the deposition of purely marine organic matter. Middle East examples include the Shilaif, Garau-Chia Gara-Sulaiy, Madbi and Naokelekan-Najmah formations. The platform/ramp is the setting for ORR deposited in neritic water depths in intrashelf basins in the Middle East. Organic enrichment occurs in the basal transgressive systems tract of a sequence, with TOC values greatest at the base, decreasing up to the MFS. Platform/ramp ORR thicken into the basin. HI values are nearly constant as the system is disconnected from the shoreline and so the organic matter is entirely marine in origin. Middle East examples occur in the Hanifa and Tuwaiq Mountain formations. The continental-margin may be the setting for multiple thick sequences of ORR. High productivity caused by upwelling oceanic currents produced fairly homogeneous deposits with high TOC and HI values. These are associated with planktonic, siliceous, phosphatic deposits. The oil shales of the Muwaqqar formation are typical of this setting.