The Study Of The Relationship Between The Tight Sandstone Reservoir Densification Mechanism And Hydrocarbon Accumulation In Southern Ordos Basin

Nowadays,tight sandstone reservoir has become one of the most important area in unconventional oil and gas exploration area.Its diagenesis and the regular pattern of the hydrocarbon accumulation are always the key questions of the research even though there are still plenty of cognition problems cannot be solved.Based on core observation,combined with analysis of conventional section,cast thin section,fluorescence thin section,SEM,particle size analysis,high-pressure mercury injection,inclusion temperature testing method,and other modern testing method,this paper made a systematic research on the relationship between diagenesis and hydrocarbon accumulation of tight sand reservoirs of Southern Ordos Basin,the types of the sweet pot of the reservoir and their development models.The following results were obtained:(1)Fully recognized that the main sandstone body of reservoir of Yanchang Formation in Southern Ordos Basin was the deposition of the delta-front facies,it mainly developed underwater distributary channel,river mouth bar,sheet sand and inter distributary bay.The average porosity was 8?16%and the average permeability was 0.05?x10-3 ?m2,so it was a typical low porosity and super low permeability tight sandstone reservoir.We think that the distributary channel and river mouth bar deposit has a batter reservoir quality and they are good sedimentary microfacies.(2)The reservoir of the study area becomes tight because of long term interaction of different diagenesis.Compaction,calcareous cementation,siliceous cementation,argillaceous cementation and asphaltization are the main reasons which cause the reservoir become tight.The protection of chlorite,the precipitation of illite and kaolinite and oil and gas emplacement can inhibit its densification process.Dissolution and such kind of effect can make the reservoir quality batter.(3)We are sure that there are three main hydrocarbon accumulation period in late J3,K1 and middle K2 to N.The first period happened in 153 to 149Ma,it can be seen as early low-maturity oil accumulation period.The second period happened in early Cretaceous,it is also the rapid subsidence stage of the basin.The third period happened after 90 Ma,the whole basin uplifted at that time and the formal hydrocarbon accumulation can be adjusted.(4)We think that the period and scale of the oil and gas filling have some impact on the reservoir diagenesis.The big scale oil and gas filling in the main hydrocarbon accumulation period inhibit the diagenesis.What is more,the oil saturation was negatively correlated with the late diagenetic responses.(5)Make sure that the time of hydrocarbon accumulation of Yanchang formation in the study area was a little earlier than when its densification happened.Its densification process has clear periodicity and its different stages caused the sandstones' early compaction,the main reservoir became tight because of cementation and the quality of the reservoir be adjusted in the late stage.The key densification was carbonate cementation in the background of compaction.Difference of the densification windows(begin with ?=15%)was mainly because of the different of its burial history.(6)We divided the types of "sweet pot" in the Southern Ordos Basin into primary,secondary and mixed.The mixed type is the majority type in Yanchang formation in study area.Meanwhile,we established the geology development model of the "sweet pot":its development and distribution are controlled by the degree of the diagenesis and sedimentation in Yanchang formation,the "sweet pot" follows the sandstones body and has a "bamboo style" distribution.