The Hydrocarbon Accumulation Conditions And Accumulation Mode Of Granite Buried Hill In Penglai9-1 Structure

The large oilfield Penglai9-1 is located on the bulge of Miaoxibei in Bohai Area, it was the ganite buried hill weathering crust reservoir of oilfield which were firstly discovered offshore China in recent years. Currently, the study on how hydrocarbon of granite buried hill accumulated in such a large-scale oilfield during the Yanshan movement is very limited.Based on tectonic and geological background of Penglai9-1 structure, this paper mainly use different methods of apatite fission track simulation, core observation, thin-section identification, logging data analysis, biomarker identification and inclusions record to analyze the tectonic evolution of the buried hill, the characteristics of granite reservoir, oil source and the conditions of hydrocarbon accumulation. Finally, the hydrocarbon accumulation process has been resumed and the mode of hydrocarbon accumulation has been established. Through researching and analysing, the main conclusions of this paper are as follows:（1）When 164 Ma ago, ganite magma intruded the metamorphic rocks using a diapir type;During 164Ma^55.053.5 Ma ago, ganite buried hill experienced a period of rapid uplift and erosion; The period of（55.0⁵3.5） Ma^34.033.5 Ma ago was a time of stable structure; During（34.0³3.5）Ma²3.3Ma ago, ganite buried hill uplifted rapidly and got eroded again; Then after 23.3Ma ago, the buried hill subsided and granite weathering crust reservoir was formed.（2）Buried hill granite weathering crust shows a clear sight of “four zonation”, from top to bottom, divided into sandy weathered zone, conglomeratic weathered zone, crack zone,fresh basement. The main space types of reservoir are intergranular pores, dissolved pores,micro-cracks and microscopic cracks. According to the thickness of reservoir, porosity and space type of reservoir, the zone has been divided into the area of reservoir developed, area of reservoir less developed and area of reservoir undeveloped. Besides, the more developed the area of reservoir is, the more thick of the total oil layers are. The area of reservoir developed mainly distributes in the southeastern region of Penglai 9-1 structure which is the high site of the buried hill weathering crust that suffered weathered and eroded most intensively. From southeast to northwest direction, the reservoir become less developed.（3）The crude oil suffered biodegraded seriously and the level of biodegradation has to be7 degree. Therefore, saturated hydrocarbon biomarker compounds cannot be used to analyze the oil source. Based on triaryl methyl steranes parameters of crude oil and oil sand samples in Penglai9-1 structure buried hill, the main oil source has to be the first and third section of Shahejie formation in the east Bohai area（Es1 + Es3 type of crude oil）. In the PL9-1-11、PL9-1-12、PL9-1-14 well area of the southern buried hill, oil came from source rocks of Doying formation（Es1 + Ed type of crude oil）.（4）Oil source of Penglai9-1 structure buried hill granite being Shahejie formation and Dongying formation in south depression of eastern Bohai area has been confirmed high-quality source rocks. In the end of Guantao formation deposition, source rock of Shahejie formation began to go into hydrocarbon generation threshold stage; In the end of Under section of Minghuazhen formation deposition, it began to enter hydrocarbon generation and expulsion heavily stage; now it has reached the peak of oil generation. In the end of Under section of Minghuazhen formation deposition, source rock of Doying formation began to go into hydrocarbon generation threshold stage; now it has entered hydrocarbon generation and expulsion heavily stage. Unconformity – fault could be the transporting system for oil of Penglai9-1 structure buried hill granite, oil migrated through "basal conglomerate interconnected pores above the unconformity" and "weathered, leached with crack- corrosion holes under the unconformity " which both are efficient migration transporting layers. Direct cap has to be the overlying Guantao strata whose bottom is the gulf between underwater distributary channel deposition. The fuchsia or purple-brown mudstone covers the unconformity and its thickness determines whether the better preservation of oil or not and the quality of oil.（5）During（55⁵3.5Ma） ²3.3Ma ago, the area of granite became a sag and granite reservoir formed by getting erosion, while Shahejie and Dongying formation deposited successively in south depression of eastern Bohai area. During 23.3¹2Ma ago, Guantao formation deposited on the buried hill and provided the direct cap for the buried hill reservoir;From 12 Ma ago to the present, the source rock of Shahejie formation began to enter hydrocarbon generation and expulsion heavily stage and the source rock of Doying formation began to go into hydrocarbon generation threshold stage. During the main period of hydrocarbon accumulation of the last 3.2Ma, oil migrated through the fault and unconformity and accumulated and finally the large-scale oilfield formed.