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 H

Ⅰ

5

—H

Ⅱ

1

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.