Sources And Accumulation Characteristics Of Mixed Oil In Drape Structure

As a result of structural framework with sag-uplift alternation, many oil-bearing drape anticline structures arrounding hydrocarbon generation sags were formed in faulted basins in eastern China. In this paper, the chemical characteristics and distribution of source-mixed oils were studied, and the sources of mixed oils, relative quantity from different sources, hydrocarbon migration pathway and accumulation mechanism were disclosed taking Gudao drape anticline of Zhanhua depression as an example using physical simulation, numerical modelling and inclusions analysis. This studies make clear where and what the petroleum exploration targets are in the future, further, to give guidances for rapid, high efficient hydrocarbon exploration and development of similar geological structures or areas.In this study, geochemical distinguishing marks, such as dinophyceae sterane, triaromatic dinophyceae steroid, 4-meth sterane,δ¹³C, and so on, are summarized systimatically among different sources in Bonan, Gunan and Gubei sub-depressions for oil-source correlation in Gudao draping anticline and its circumambience. Correlation results show that there are three types of oil and come from Bonan, Gunan and Gubei sub-depressions respectively. Additionally, oils in different areas around Gudao swell were analyzed. The northwestern area's oil comes from Bonan sag's source rocks of the 4th member of Shahejie formation(Fm.) (Es₄) and the 3rd member of Shahejie Fm. (Es₃), and northern area from Gubei sag's source rocks of Es₃; and southern one from Gunan sag's of Es₃ and the 1st member of Shahejie Fm. (Es₁), southwestern slope area from Gunan sag and Bonan sag besides some deep strata's oil only from Bonan sag's source rock. According to maturity and geochemical signatures, these oils in southwestern slope area are divided into two types and four kinds.Two-end mixed source oils experiments based on subtle oil-source correlation results provides a great many of quantificational distinguishing models for discovered reserves in Gudao swell and its circumambience. Experiments shows that about 62% of discovered reserves comes from Es₃ source rock of Bonan sag, and about 10% from Es₄ source rock of Bonan sag, and about 10% from Es₃ source rock of Gunan sag, and about 8% from Es₁ source rock of Gunan sag, and about 10% from Es₃ source rock of Gunan sag.A new classification on oil and gas transport system was put forward after ascertainment of transport factors and assembled relationship. As an important factor for hydrocarbon migration, faults were studied considering those action for oil migration or seal ability. Studies indicates faults are open and migrates oil and groundwater when fault activity speed (V_f)≥3.4m/Ma in Dongying Fm. or V_f≥1.9m/Ma in Guantao Fm., or SGR≥0.40 in any stratum. On the contrary, faults are close and be closed reserves.Analysis by buried history and fluid inclusion shows that there three times of hydrocarbon accumulation at the end of sedimentation of Dongying Fm., during the sedimentation of from the end of Gutao Fm. to early Minghuazhen Fm. and the end of sedimentation of Minghuazhen Fm.. Hydrocarbon migration simulation shows clearly superior migration routes of three sand-rich strata during three main stages of hydrocarbon migration and accumulation. Southwestern part, northeast partnear Gunan fault and northern part near Gubei fault are the main entrances into Gudao swell.In studied area, pools charaterized by many sources and mixed source oils are widely exist beacause of structural pattern alternated with sags and swells, oil and gas reserves types and accumulation efficiency are controlled by transport system's type in different structural belts, and complex and episodic accumulation process is affected by multistage migration based on abnormity pressure's episodic discharge. For those reason, two hydrocarbon accumulation models, i.e. actic region hydrocarbon accumulation model near main fault belts and gentle slope hydrocarbon accumulation model are summarized.