The Distribution Of Elements,Carbon And Oxygen Isotopes On Marine Strata And Environmental Correlation Between They And Hydrocarbon Source Rocks Formation—An Example From Ordovician Basin,China

Lower Paleozoic marine deposits that commonly displays high-over maturity (Ro ranging 1.3 %⁴.5%) and lower total organic carbon contents (TOC were widespread in China, where were further complicated to distinguish the efficient hydrocarbon source rocks because of TOC , H/C, etc. have been changed by a long thermal evolution. Currently, the accurate evaluation of validity for marine hydrocarbon source rocks is one of the pending important problem in oil and gas exploration and developments. Some scholars had discussed different restoring methods for hydrocarbon generation potential of high-over mature potential source rock by many methods of geochemistry, but those have some difficulties and differences.The developments of hydrocarbon source rocks are primarily controlled by sedimentary environment that including ( i) productivity in the surface waters, (ii) sediment accumulation rate, (iii) bottom-water oxicity, and (iv) organic-matter source, these can be accepted as key variables. The chemic stability of trace elements and carbon isotope composition are stronger than organic matters even in high-over mature stage. Therefore, the inorganic parameters can show original sedimentary environment reliably, and have more predominance to further differentiate efficient hydorocarbon source rock in high-over mature stage. The Ordovician strata distribute widely in the West-South area, Ordos Basin. The Majiagou Formation (O₁m) in this area is made up of a suite of platform carbonate sediments, whereas the Wulalike Formation (O₂w) consists of a suite of foreslope gravity-flow sediments including clastic rocks intercalated with mudstone and carbonate rocks. In this paper, the paleoenvironments of potential hydrocarbon source rock of these Formations is discussed using inorganic parameters. The aim of this study was to grope for new approach for distinguishing the efficient hydrocarbon source rocks in high-overmature marine deposits.1. The main oxides, trace elements, rare earth elements and carbon,oxygen isotope compositions display a regular temporal and spatial evolution. The cluster and correspondence analytical results show that geochemically the different formations in the researchful area are not only independent but also correlated. Vertically, the geochemical signatures of these strata also exhibit an episodic and cyclic pattern of evolution, i.e. the simultaneous increase or decrease in most of the elements and parameters. In this respect, the evidences in favour of the above-mentioned regular evolution in time and space are strongly supported by the macroscopic features and depositional environments of these strata.2. High contents of Barium, Positive 513Ccarb excursions have reflected a high paleo-productivity and burial amounts of organic matter in Oik, Oaw in West and Oim6 Formation of Well XT1 that contain 0.2% total organic carbon, and have a good type of organic matters, high hydrocarbon potential. So these Formations can be regarded as efficient hydrocarbon source rocks. However, in O\z, O\s in West and Dim4"6Formations of Qishan section, TOC<0.2%, Ba<25 (xlO'6) , 613Ccarbmost have negative values, which indicate a low burial quantity, unfavorable preserved conditions, and low-quality of organic matters. These suggest that carbonate rocks in such environments are poor hydrocarbon source rocks. Rare-earth elements and oganic matter, both have osculatory correlation, REE fractionating degree can reflect sedimentation rate. Poor source rocks are rapidly deposited in shallow water, but efficient hydrocarbon source rocks are slowly deposited in bottom-water anoxicity, stagnated, bathyal environment mainly, black shales also can develop the efficient hydrocarbon source rocks, which sedimentation rate is high part. Therefore, paleo-productivity, burial amounts of organic matter, and sedimentation rate can be rebuilded effectively using inorganic parameters ?Ba, stable carbon isotopic composition and rare-earth elements, etc, and can be applied in further estimate studies for the validity of...