Accumulation Conditions Research On Tight Oil Reservoir Of Chang 7 Member Of Yangchang Formation In Ordos Basin

Tight oil is an important component of unconventional petroleum,which has become a focus of petroleum exploration and development after the successful development of shale gas.Chang 7 tight reservoir deposited under the delta front and shallow-deep lacustrine turbidity current environment.It had large areas of oil but not obvious oil-water boundary,which located in the organic-rich shale layers.In previous studies,Chang 7 was regarded as a main source rock of Yangchang Formation,and its accumulation condition and process is mostly failed to be researched systematically.Research on source rock,reservoir and its relationship is to figure out Chang 7 accumulation characteristics and their control factors.Pore evolution,history of hydrocarbon generation,accumulation periods and accumulation dynamics were studied.The accumulation process was summarized and the accumulation model was established in the Chang 7 tight oil reservoir.Results show that a set of semi-deep and deep lacustrine high-quality source rock developed in the Chang 7;the organic matter aboundance is high with average TOC value of 6.75%;the kerogen is of type ?-?1 and the organic matter has entered the mature stage with average Ro value of 0.85%.Tight reservoir which formed under the delta front and shallow-deep lake turbidite environment contacts closely with high-quality hydrocarbon source rocks,forming good source store configuration.Chang 7 tight reservoir is characterized by fined-size and high matrix,feldspar and lithic fragment coments;porosity and permeabillity is low with porosity ranging from 6% to 14% and permeability generally below 0.3×10-3?m2.The major type of pores of the reservoir are the secondary dissolution pores,with primary remanent intergranular pores developing locally and clay mineral intercrystal pore existing extensively.Scanning electron microscope,nitrogen adsorption,mercury injection pressure,and nclear magnetic resonance(NMR)are used to evaluate reservoir pore structure characteristics.It is showed that pore throat is narrow,with pore size less than 30?m and pore throat less than 1?m;and micro pores volume within 300 nm account for about a quarter of the total pore volume,mainly composed of clay mineral intercrystal pore and dissolution micro pore.The information of pore and throat is reflected in different aspects through these methods,which play a complementary role in evaluation of pore structure.In early Cretaceous the medium-strong compaction caused by rapid burial and the late iron carbonate cementation are the main reasons for the reservoir densification,while pore space is mainly composed of dissolution pore because of acidic fluid dissolution which expelled from source rock along with mature process.Reservoir have developed two phases oil and gas inclusions.Combined with the history of pore evolution,hydrocarbon generation and accumulation of dynamic analysis,it is confirmed that early immature oil accumulated in a small-scale in late Jurassic-early Cretaceous.Mature oil acuumulated massly with overpressure derived form hydrocarbon generation and undercompaction,although reservoir has small pore throat and huge resistance in the end of early Cretaceous.