Research On Reservoiring Mechanism Of Tight Sandstone Of Sha 1 Member In The Beach Area Of Nanpu Depression, Bohai Bay Basin

The coupling relationship between sandstone reservoir densification and hydrocarbon accumulation is the key scientific problem of controlling tight oil reservoir formation.Aiming at this essential problem,taking tight sandstones within Sha 1 Member from Beach area of Nanpu Depression in the Bohai Bay Basin as the research object,Reveal the densification mechanism of sandstone reservoir by porosity predictive model based on the research on the characteristics of densification and diagenetic evolution.Combing the analysis of tight oil reservoir characteristics,compactification history,oil accumulation and the relationship between reservoir densification and hydrocarbon accumulation,the process and mechanism of oil migration are discussed.And tight oil accumulation models are built.As the depth increases,compaction makes pore structure worse.It is a direct factor of porosity decreasing.While cementation is the key reason of reservoir densification,further decrease the porosity of reservoir.The compaction timing of sandstone reservoirs was about10.5Ma.By using fluid inclusion analysis,the hydrocarbon accumulation history shows two hydrocarbon fluid flow stages in Sha 1 Member.The first hydrocarbon charging stage is from Late Guantao to Early Minghuazhen(from 15.5Ma to 12.0Ma),and the second is in the Middle-late Minghuazhen(from 7.0Ma to 3.0Ma).The first oil charging happened before Sha1 member reservoir densification.And the second charging started at Middle-late Minghuzhen after the reservoir densification.The matching relationship between reservoir porosity evolution history and oil and gas filling history showed the tight oil reservoir accumulation mechanism was"the densification earlier than the accumulation".Based on logging electronic response,reservoir types,hydrocarbon generation index and exploration achievements,the tight reservoirs were divided into 3 types,of which the 1st class and the 2nd class tight reservoirs were fracture-pore type with good hydrocarbon shows;the3rd class tight reservoirs with limited fractures had poor hydrocarbon shows,reflecting that the fracture-developed zones within tight reservoirs were favorable directional zones for the exploration of tight oil.Characteristics of hydrocarbon migration and accumulation dynamics showed that Overpressure difference between layers provided the main force for the tight oil and charge the main migration resistance of oil was capillary resisting force.There existed three vertical superimposed pressure belts which were"the upper hydrostatic pressure zone"and"the middle abnormal high pressure zone"and"the lower abnormal high pressure zone"from beach area of Nanpu Depression.Sha 1 Member which in the the lower abnormal high pressure zone,the surplus pressure(between 10MPa to 20MPa)were much bigger than the displacement pressure.Overpressure difference between layers affected the enrichment degree and filling area,and the distribution belt which has big difference of overpressure between layers might be the favorable area for tight oil enrichment.Hydrocarbon expulsion and its charge in tight sandstones are greatly promoted by pathway systems consisting of tectonic and hydraulic fractures in mudstones and interfaces between sandstones and mudstones of Sha 1Member and the Nano-sized pores inside the mudstones and kerogen network linked up with fractures.The study of main controlling factors of the tight oil accumulation showed that the widespread high-quality source rock provided solid material basis for the formation of tight sandstone oil;good matching relationship between tight sandstone reservoirs and source rock,and induced fracture systems provided reservoir space for tight oil;the overlying stable semi-deep lacustrine strata depositing thick mudstone and silty mudstone provided good sealing conditions for tight oil;residual pressure provided stable reservoir-forming dynamics for tight oil.There are two kinds of tight oil accumulation models in Beach area:(1)inside source rock-overpressure filling-pore and induced-fracture type tight sandstone accumulation model;(2)near source rock-overpressure filling-fault-sand body transport system-pore and induced-fracture type tight sandstone reservoir accumulation model.The tight sandstone oil resource quantity of Sha 1 Member in Beach area of Nanpu Depression was evaluated by using volumetric method for small areas and analogous method for abundance of resource,the tight oil resource quantity was 1.11×10~8t and the recoverable resource quantity was 0.089×10~8t.Comprehensive assessment showed that the study area was classified as class?tight oil province,and it was considered that the favorable sweet spot target area was located in NP3-19-NP3-20 well area within No.3 tectonic belt of Nanpu Depression with sweet spot target area covering 40.5km~2.