FORMATION CHARACTERISTICS OF LARGE-MIDDLE LITHOLOGIC-STRUCTURAL COMPOSITE OIL AND GAS FIELDS IN THE QIAN'AN-CHANGLING DEPRESSION OF THE SONGLIAO BASIN

Many sets of high-quality source rocks and delta-front-facies sandstone reservoirs were developed in the Lower Cretaceous of the Qian an-Changling Depression in the southern part of the Songliao Basin. And upper, middle and lower three sets of pool-forming assemblages were formed. The Huazijing Terrace in the eastern part of the Depression possessed various advantageous conditions to form large-middle struc-tural-lithologic composite traps: a) multiphase delta-front-facies areas were intersecting and superimposing each other, and the lateral facies changes and updip thinning-out of lithology often occurred ; b) the diffe- rentiation of south and north inherited sedimentary microfacies generated on the background of paleotopog-raphy-the buried paleouplift zone in Qiannan area; c)paulopost tectonic movement resulted in the elevation of the East Songliao Basin, and made the eastern sandbodies mass updip thinning out; d)paulopost tectonic movement resulted in a series of minor faults, screening off the updip direction of sandbodies; e) large scale of oblique water current formed organic allocation with regional structural patterns; and f) dia-genesis resulted in the compaction of reservoirs. Lithologic-structural composite traps formed in the Nen-jiang and Mingshui perieds, and matched well with large scale of hydrocarbon migration periods. This provided good conditions to form a group of large-middle structural-lithologic composite oil and gas fields in the Huazijing Terrace.