Abstract: Taking the Longfengshan sub-sag in the Changling fault depression, southern Songliao Basin, as an example, this paper discussed the hydrocarbon accumulation conditions, including source rock, reservoir, cap rock, and hydrocarbon migration and preservation. It is considered that the Changling fault depression has the basic petroleum geological conditions for hydrocarbon accumulation, with both vertical and lateral pathway system constituted by fault and sandbodies. Furthermore, deep structure was shaped earlier than the main hydrocarbon accumulation stage, which is favorable for hydrocarbon migration and preservation. There are four main controlling factors for gas accumulation in the Yingcheng Formation clastic rocks in the Changling fault depression. First, quality and thermal evolution degree of the Shahezi Formation and Yingcheng Formation source rocks in deep sags controlled the oil and gas source supply. Second, predominant sedimentary microfacies controlled the distribution of favorable sandbodies while reservoir properties determined the hydrocarbon enrichment degree, and presence of secondary pores significantly contributed to the gas-bearing potential. Third, the stable mudstone cap rock in the upper Yingcheng Formation and the faults developed in late stage controlled the preservation conditions and determined the major hydrocarbon-rich strata. And fourth, the fault-sandbody pathway system formed under the structural setting in hydrocarbon accumulation stage effectively controlled the oil and gas migration direction. Study in this paper suggests that the clastic rocks in the Changling fault depression have a gas accumulation pattern of “source supply from deep sag, conduction via a vertical and lateral fault-sandbody pathway system, and structure-cap- rock controlling hydrocarbon accumulation”.

Key words: southern Songliao Basin, Changling fault depression, Longfengshan sub-sag, clastic rocks, gas, hydrocarbon accumulation conditions, main controlling factors