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Origin-Based Classification Scheme for Fine-Grained Sediments: A Case Study From the Eocene Green River Formation in Uinta Basin

Abstract

With the recent dramatic development of unconventional energy exploration and production, the topic of fine-grained sediment has emerged as one of the latest hotly debated topics on sedimentary processes and environments. Although there are numerous classification schemes that exit for fine-grained sediment, most of which are based on the relative proportions of different components, only a few are based on origin. This study defines a detailed origin-based classification scheme for fine-grained sediment using field-scale log data and micro-scale thin section petrographic data. The proposed classification scheme, which is based on fine-grained sediment origins, characteristics, transport and deposition processes, is based on a ternary diagram. The primary classes are bed-load facies, suspended-load facies and biochemical facies. The bed-load facies, which is mainly deposited by traction, in channel axis during waxing phase of flood, are mainly composed of sandstones and pebbly sandstones. In proximal levee, it is deposited by traction, with minor fallout reconcentration, and mainly composed of fine to coarse sandstones with silt levels. The suspended-load deposit, which deposited by traction and fall out, with minor sediment gravity processes(debris flow), mainly accumulated at channel margin to distal levee in waning phase of the flood. Lithologically, it is composed of sandstones and pebbly sandstones. In distal levee, it is deposited by fall out, common with reconcentration and flocculation, and mainly composed of fine sandstones and. The biochemical facies, which are deposited by a continuous shower of mostly organic detritus falling from the upper layers of the water column, consists of shale with different proportions of organic matter. The main mechanism for deposition of this group are flocculation, fallout and reconcentration of biological debris and fine grained silicates, with flocculation a frequent mechanism in environments with fluctuating salinity. Its occurrence is may be more a function of accumulation rates of organic versus terrigenous matter (or river flood deposit) than local biological productivity. Transitional zone is dominated by organic-rich silt-stones, which are deposited by biological and biochemical processes, fall out, flocculation and reconcentration. The biochemical facies can be divided into three subtype as organic matter facies, calcareous facies and other facies. Different group of fine-grained sediment plays different role in unconventional petroleum system, with river flood deposit and chalky carbonate as the favorable reservoir, and organic-rich deposit as the mainly source rock. The proposed origin-based classification may help provide a better understanding of depositional conditions and identify target zones for unconventional resources both of shale gas and tight gas.