As a new field of unconventional oil and gas exploration and development in China,shallow gas is large in resource potential.Various data including core,well logging,3D seismic and laboratory test data were integrated to study the shallow gas reservoir types,distribution characteristics,genetic mechanism and main controlling factors of shallow gas enrichment.The following results were obtained.The gas traps are controlled by anticline structure background and segmentation of multiple faults and there are mainly 4 types of gas reservoir traps,including anticline,faulted-anticline,fault-block and fault-nose.And the areal distribution of gas reservoirs is controlled by the positive structures and they mainly occur at the high positions of anticline and nose structures.Vertically,the single layer HⅠ5—HⅡ1,which is in the middle and lower parts of formation,is the main gas bearing interval affected by reservoir-cap assemblages.Gas in the Putaohua structure is mainly of biological genesis and is sourced from the first and second members of the Nenjiang Formation.In contrast,gas in the Puxi nosing structure is generated from the first and second members of the Nenjiang Formation,Qingshankou Formation and deeper formations,and is a mixture of biological gas,oil-type gas and abiogenetic gas.On this basis,the main controlling factors of shallow gas enrichment in Heidimiao reservoirs are summarized as follows:①multiple sets of good hydrocarbon source rocks offer a material basis for shallow gas;②reasonable timing of gas source faults and effective cap rock controls the horizons of shallow gas accumulation;③sandbodies of fluvial-dominated delta front act as high quality reservoirs;④lateral sealing capacity of fault ddetermines enrichment scale of shallow gas.Finally,the shallow gas accumulation pattern is established and showed as follows:tectonic reverse happened at the end of the Mingshui Formation stage,and the boundary faults in the areas with densely distributed faults were activated and opened,so that gas generated by the underlying source rocks migrated upward along the source rock-rooted faults into the high quality reservoirs connected well with the source rock-rooted faults,such as under water distributary channel,mouth bars and distal bars.After a short distance lateral migration in the high quality reservoirs,the shallow gas accumulated in anticlinal traps,fault-anticline,fault-block and fault-nose traps.The cumulative geological reserve of shallow gas was calculated to be 8.68 BCM by using volumetric methods on the basis of gas logging interpretation and 7 potential target areas were identified.
