| The Mid-Lower Yanchang formation has recently been an emerging appreciation in deep-seated oil exploration and development in Jiyuan oilfield,the central-western Ordos Basin.Tiebiancheng area is located at the southeast of Jiyuan oil field,where several drilling tests produced industrial flows from Chang 8 and Chang 9 oil-bearing layers,such as J51 and W554 drilling wells,etc.,showing a good potential for oil exploration and development in the Mid-Lower Yanchang formation.However,there are still some confusion and controversial issues on their reservoir properties,oil accumulation mechanism and enrichment distribution,which restricted the oil exploration and development process of the Mid-Lower Yanchang formation in this area.Here,constrained by drilling core description and core sample testing,the methods of well-logging interpretation,reservoir prediction and evaluation integrated with geological modeling,are carried out to reserach the sedimentary source,sedimentary microfacies,facies controlled sandstone reservoir features,and the relationship among lithology,physical property,oil content and electricity,as well as the oil reservoir identification criteria of the Chang 8 and Chang 9 oil-bearing layers in Tiebiancheng area.Additionally,integrated with analyses of the oil and source correlation,the temporal relationship between diagenesis and accumulation,and the excess pressure difference between source and reservoir layers,the main controlling factors and favorable area distribution of the Chang 8 and Chang 9 reservoirs in Tiebiancheng area are comprehensively analyzed and discussed.Accordingly,the main results and some new understandings achieved in this study can be summarized as follows.?1?The main source system and sedimentary microfacies of the Chang 9 and Chang 8oil-bearing layers in Tiebiancheng area are made clearly by the analysis of clastic minerals,grain size imaging and core description-plots of logging facies.It is believed that Chang 9 and Chang 8 oil-bearing layers are mainly controlled by the NW-SE?Yan-Ding?braided river delta sediment source system,which mainly produces two kinds of sedimentary microfacies,i.e.underwater distributary channel and distributary bay near the end of the braided river delta front.Among them,the upper layer of the Chang 9 oil-bearing layers is mingled with limited dark mudstone deposit of semi deep lake facies in the southwest of the study area,and the southeast part of the Chang 8 oil-bearing layers is mingled with deposition of predelta subfacies.?2?The comprehensive analysis of core test,logging interpretation and oil production test data reveals that Chang 9 sandstones in the study area mainly belong to ultra-low permeability reservoirs,with a porosity distribution of?7-14?%and an average value of10.16%,and a permeability distribution of?0.05-3?×10-3?m2 and an average value of 0.46×10-3?m2,corresponding to the lower limits of effective reservoir porosity,permeability and oil saturation parameters as 8.0%,0.1×10-3?m2 and 50%,respectively.Chang 8 sandstones blongs to typical tight reservoirs with a porosity of?4-10?%and an average value of 6.98%,a permeability of?0.01-0.3?×10-3?m2 and an average value of 0.112×10-3?m2,corresponding to the lower limits of effective reservoir porosity,permeability and oil saturation parameters as 6%,0.05×10-3?m2 and 31%,respectively.?3?The study of reservoir petrology and diagenetic pore evolution shows that the Chang 9 and Chang 8 sandstones,deposited near the end of delta front in Tiebiancheng area,are of fine grain,low quartz content,high content of feldspar and plastic debris,experienced strong coMPaction?porosity reduction rate up to 61?67%?,strong late cementation of carbonate and illite?porosity reduction rate is close to 18?28%?,and weak medium-term dissolution?porosity increase rate is 5.1?8.2%?,which resulted in the ultra-low permeability and tight reservoir conditions at the end of the Early Cretaceous sedimentary burial stage,approximately approached to present sandstone physical properties.?4?According to the GC-MS test data of source-rock and crude oil samples as well as the oil-source correlation analysis,it is revealed that the abundance of 17??H?-C30 rearranged hopane(C30*)is very low,C30*/C30 hopane is only 0.08,C29Ts/C29 hopane is as low as 0.42 in Chang 8 reservoir crude oil,which is mainly belongs to class I crude oil from Chang-7 oil shales;the C30*abundance of Chang 9 reservoir crude oil is high,C30*/C30 hopane is close to0.28,C29Ts/C29 hopane is 0.77,which shows the characteristics of type I-II I-II mixed crude oil contributed by the mixing of Chang-7 oil shales and chang 9 dark mudstones,which also indicates that the dark mudstones in the upper part of Chang 9 oil-bearing layers has a certain contribution to hydrocarbon accumulation.?5?The diagenesis and accumulation process,the driving force of source reservoir pressure difference and the prediction of favorable reservoir forming areas show that there are two types of reservoirs in the study area,i.e.,ultra-low permeability tight lithologic traps and low amplitude nose like structural lithologic traps.The hydrocarbon accumulation and diagenetic densification of the reservoirs are nearly synchronous in the maximum burial warming period?123?105?MA in the middle and late Early Cretaceous.The distribution is controlled by the distribution of effective reservoir desserts and the relatively high excess pressure difference?>5.0MPa?between source and reservoir. |
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