Source Rock Deposition Controlled by Tectonic Subsidence and Climate in the Western Pearl River Mouth Basin, China Evidence From Organic and Inorganic Geochemistry

Abstract

Abstract

Lake deposits, which contain significant petroleum resources, are important to understand and predict. The deposition of lacustrine source rocks, unlike marine source rocks, displays strong heterogeneity, which is caused by its small water reservoir size and frequent external environment changes. The strong heterogeneity can be evidenced from wide ranges of salinity, pH, Eh and marked variation of biota and can be traced by various geochemical parameters. Sometimes these indexes, however, are contradictory and thus restrict source rock evaluation.

In this study GC-MS and trace elements analysis were conducted by using 20 and 51 source rock samples respectively from the western Pearl River Mouth Basin. The co-variation of organic and inorganic indexes indicate the combination is a valid method to reconstruct primary productivity and depositional environment. The synchronous changes in organic matter input, primary productivity and sedimentary environment were reasonably interpreted by two deposition models, which were controlled by the tectonic subsidence and climate.

During the E₂w depositional stage, the rapid subsidence and low sediment supply led to thick sediments deposited in deep lacustrine environment and resulted in sediment starvation. The low water injection provided little terrigenous organic matter and oxygen. Besides, the small area/depth ratio impeded the water circulation, thus resulted in shallow thermocline and euxinic, anoxic bottom environment. Therefore abundant algae, which contributed to the high AOM content and high productivity, can be preserved. The warm and wet climate gave birth to autochthonous organism, such as dinoflagellates and pavlova gyrans. During the E₃e depositional stage, slow subsidence and sufficient sedimentary supply resulted in expanding, shallow lacustrine environment. The greater area/depth ratio and high water inflow made environment unstable. Enough TOM was transported to the slop but little to the depocenter. The hot and dry climate led to decreasing autochthonous organism and evaporation environment. The strong evaporation and high water flow resulted in saline, suboxic-dysoxic acid bottom environment and deep thermocline.

Different sedimentary environments resulted in different geochemical features, thus using the various geochemical features can trace palaeosedimentary environments. Furthermore, potential source rock deposition can be predicted by using suitable models when detail data is not available.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016