Mechanism Of Oil And Gas In The Pucheng Oilfield Of Dongpu Sag

Dongpu Sag is a typical Tertiary saline lacustrine Depression,in which Pucheng Oilfield is the second largest oil-gas field.The origin and mechanism are complex.The geochemical methods such as Rock-Eval,GC-MS and compound specific carbon isotopic analysis combined with geological were used to unravel geochemical and hydrocarbons generation and accumulation features of the crude oils in the Pucheng of the Dongpu Sag.The characteristics of crude oil in Pucheng of Dongpu Depression are remarkable:?1?crude oil has typical saline lacustrine characteristics,such as predominance of phytane to pristine,C₃₅-hopane to C₃₄-hopane,enrich gammacerame,?-carrotane and Long-chain n-alkanes(n C₃₇ and n C₃₈);?2?The majority of the crude oils discovered are less mature oils situated in the middle-shallow horizons while the minority of the oils discovered are high to over-high mature,located in deep horizons;?3?The compound specific carbon isotopic in crude oil shows two-stage distribution,which is similar to that of saltwater lacustrine crude oil.According to maturity parameters,crude oils can be divided into two types,Class I crude oils from the first member of Shahejie Formation to the middle member of Shahejie Formation,and Class II crude oils from the lower sub-member of Shahejie Formation to the upper sub-member of Shahejie Formation.Oil source correlation shows that Pucheng crude oil has good comparability with source rocks buried deeper than 3000m in Pucheng sag and Puwei sag.The oil in Es3s has strong comparability with that in Es₃^z.The oil in Es₃^s and Es₃^z were mainly derived from the Es₃^z source rocks with a burial depth>3000m in the Pucheng and Puwei sub-sages.The Es₃^s,Es₃^x and Es₄^s source rocks also were contribution to hydrocarbon generation.Hydrocarbon accumulation features for the crude oils in the Pucheng mainly include:?1?Types of the oil reservoirs are diverse,including fault blocks reservoir,fault-lithologic reservoir;?2?The reservoir is generally thin single layer reservoirs,with the majority of extra-thin and thin layers,high shale and carbonate content of reservoirs;The low porosity,ultra-low permeability and low permeability reservoirs are the main reservoirs in the middle and deep layers of Pucheng;?3?Two stages of hydrocarbons accumulation with the early one dominated.?4?There are two genetic mechanisms that responsibal for the immature oils in northern saline lacusturine:biolipids and sulfur-rich macromolecules for early hyduocarbon generation.?5?local gypsum-salt rocks in the western wing of Pucheng control the temperature of oil and gas pools.Pressure and hydrocarbon migration,deep oil and gas reservoirs partially undergo TSR.The hydrocarbons migration and accumulation models was established:"early generation,near source-fault transmission-fault lithologic reservoir-model".