中国古第三紀南嚢湖成堆積盆地の石油地質地化学

Abstract

The Paleogene to Neogene Nan Xian sedimentary basin, which is typical of non-marine oil producing sedimentary basins in eastern China, is composed of four structural depressions called the Bi Yang Depression, Nan Yang Depression, Xiang Yang Depression, and Shuang Gou Depression, respectively. The northern Bi Yang and Nan Yang Depressions are the major oil fields in Nan Xiang sedimentary basin, where 3×109 ton of crude oil has been produced during the 40 years since the start of petroleum exploration in the 1950's. Nan Xian crudes are generally characterized by high wax contents (30 to 50%), high pour points (40 to 50°C), and low sulphur contents (0.05 to 0.14%), suggesting that the crude oils are derived from biodegraded higher plants. Some biomarkers such as steranes and hopanes in crude oils, however, suggest significant contribution of zoo- and phytoplanktons.Oil/source rock correlations by biomarkers showed that black shales of middle Paleogene He Tao Yuan Formation are major source rocks in Nan Xiang sedimentary basin. The He Tao Yuan Formation is characterized by a sequence of fluvial alluvial, delta front, and deep basin facies in lacustrine depositional environment. The He Tao Yuan black shale was formed by turbidity current deposition in a deep lake basin when great amounts of river borne clastic sediments accumulated during the marked subsidence of the basement. The He Tao Yuan black shales were enriched in organic matter (present average total organic carbon content=1.8%) because of the increased primary production due to temperate climate and the anoxic depositional environment in the deep basin center. The kerogen of the He Tao Yuan black shale is characterized by oil prone Type I and Type I-II kerogens. The Bi Yang Depression and a part of Nan Yang Depression, especially, could possess the most favorable depositional condition for the formation of petroleum source rocks among others. Several reservoir structures were formed during the uplift of basement during late Paleogene to early Neogene time. Reservoir rocks are mainly sans nes of He Tao Yuan Formation deposited in fluvial and alluvial planes, and delta front in Nan Xiang paleo-lake. He Tao Yuan source rocks have been maturated enough to produce hydrocarbons with the re-subsidence of basement in Neogene Tertiary time. Lithofacies of turbidite alternation was advantageous for the effective expulsion and secondary migration of petroleum hydrocarbons. Much of the hydrocarbons generated have been efficiently accumulated into previously formed traps of the same stratigraphic horizon to form petroleum deposits. The Bi Yang and Nan Yang Depressions have mainly produced liquid oils with a little gas of biodegraded origin. In addition to shallow reservoir depth and oil prone kerogen types of source rocks, the low gas/oil ratio in Bi Yang and Nan Yang oil fields is mainly due to that the He Tao Yuan source rocks have not been overmatured to generate thermocatalytic gases through the evolution history of Nan Xian sedimentary basin.