贵州东部若干古油藏的形成和破坏

The Formation and Destruction of Palaeo-Oil-Reservoirs in the East of Guizhou Province

本文根据实际资料论述了黔东下古生界若干古油藏的形成条件和破坏因素,说明原始石油地质基本条件优越,地史上曾经形成大规模油气聚集的事实.但是,由于后期的破坏,揭示出演变和保存这两个不可忽视的重要问题.这不仅对黔东,而且对我国南方下古生界海相沉积区突破油气关起着显著的控制作用.指出今后对下古生界油气普查勘探工作应着重目的层尚处于地腹地区的演变和保存条件研究.

In the east of Guizhou, paleo-oil-reservoirs have been separately found in Majiang, Ongan, Tongren-Wanshan and Danzhai. What is called a paleo-oil-reservoir means what was formed in the earth's history but destroyed afterwards. The paleo-oil-reaervoirs we can see now are in fact the metamorphosed bitumens in solid state, which are the residual of destroyed paleo-oil-reservoirs. The paleo-oil-reservoir layers are sandstone and carbonatite strata of the early Paleozoic. The paleo-oil-reservoirs in sandstone have intergranular space and those in carbonatite have corrosional holes, crystal holes, intercrystal voids and interlamellar crevices, etc. It is estimated that there are ten million tons of Tongren-Wanshan paleo-oil-reservoir residual bitumen and thousand million tons of Majiang paleo-oil reservoir residual bitumen, which shows us that there were once one hundred million, tons of crude oil in reservoir in Tongren-Wanshan and 16 hundred million tons of crude oil in reservoir in Majiang. In terms of the comparison between organic geochemical oil source parameter data and spectrum drawing, the oil source rock were mainly mud oil-originating rocks of the early Cambrian and secondly were mud oil-originating rocks of the early-middle Silurian ( Majiang paleo-oil-reservoirs ) and the carbonatite oil-originating rocks of the early Cambrian Based on the calculation of the time and temperature index ( ΣTTI ) and the results obtained through organic geochemical analyses, it is shown that oil was chiefly formed in the middle-late Caledonian period, identical with the Caledonian movement ( the Duyun movement at the end of the middle Ordovician and the Guangxi movement at the end of Silurian ) . That is, the oil-forming period of oil source rocks was closely related to the paleostructure in the Caledonian period, and the Caledonian movement made oil and gas move, gather and finally form paleo-oil-reservoirs. The Guangxi movement made the crust ascend too high, causing the paleo- oil-reservoirs in Majiang, Tongren-Wanshan and Danzhai to be exposed, corroded and dissipated, and the oil remaining in the reservoirs was oxidized and degenerated. From Variscian to early Yanshan, deposition was made and the upper cover of reservoir was more than 500m thick; the burying temperature was 154℃-196℃; the organic changing degree was up to the highest and oil was metaphosed and changed to dry air and metaphosed bitumen, with the reflecting rate ( R° max%) greater than 2.1-5.75. The Yanshan movement again caused the paleo-oil^reservoirs in the above places to be exposed, corroded, changed and dissipated. The discovery of the paleo-oil-reservoirs in the east of Guizhou shows that there are advantageous geological conditions for crude oil to form in the car-bonatite depositional regions in the east of Guizhou and even in the south of China and that in the earth's history oil and gas were once gathered on a large scale. The destructive factors of the paleo-oil-reservoirs-development and preservation-should be considered in discovering oil and gas in the carbonatite regions in the south of China, and researches on the preservation conditions of objective strata in the hinterland should be made.