The Reconstruction Of Paleo-fluid Potential And The Study Of Oil Migration And Accumulation Characteristics Of The Third Member Of Shahejie Formation, Dongying Sag

Dongying Sag, one of the richest petroliferous sags in east China, with the proved reserves23.4×10⁸t, is the biggest lithologic reservoir distribution area in Jiyang depression. The potentialreserves of lithologic reservoirs here are considered to beyond 3×10⁸～4×10⁸t. Lenticularsandbody reservoir is an important type of lithologic reservoirs. However, the filling andaccumulation mechanism of these reservoirs are complicated, and the petroliferous property ofthe sand lens is quite different. This dissertation, based on the results of predecessors' research: (1)by integrating the systematic fluid inclusion analysis with delicate burial history analysis,analyzed the phases of hydrocarbon charging and determined the charging time of Dongying sag;(2) by using the fluid inclusion PVT modeling, reconstructed the paleo-pressure of hydrocarboncharging; further, based on the testing datum of present formation pressure, analyzed theevolution of paleo-pressure and predicated the distribution of present formation pressures; (3)based on the former analysis, reconstructed the paleo-fluid potential of different periods ofhydrocarbon filling and accumulation; (4) built the migration passage framework in subsags,Niuzhuang, Lijin and Boxing, respectively; (5) at last, on the basis of the results ofaforementioned research and the analysis of the porosity and permeability of sand bodies,discussed the process of hydrocarbon charging and the controlling factors of oil saturation of thesand lens of the third member of Shahejie formation in Dongying sag. The main conclusions areas follows:Three events of hydrocarbon charging had occurred here: the first phase is between 34～24Ma, the second is between 13.8～8.0Ma, and the third is between 8.0～0.0Ma, respectively.The oil charge began about 34 Ma; the main periods of hydrocarbon charging, however, were inthe second and the third stage. Especially, the ultimate distribution of oil and gas here iscontrolled by the third hydrocarbon charging. The uplift of strata, because of the Dongyingmovement in the late Oligocene, resulted in the temperature of source rocks decreased, and thenrestrained the hydrocarbon generation. Consequently, no oil accumulated in the sandbodiesbetween 24～13.8Ma. What's more, three stages of hydrocarbon charging were observed in Niuzhuang and Lijin subsag, but only the third phase was detected in Boxing subsag.Overpressure in the Paleogene strata of Dongying sag is extraordinarily obvious. The pressure offormations above the second member of Shahejie formation is normal. The pressure below thethird member of Shahejie formation was generally developed in abnormal high-pressure system.The overpressure of Es₃^s is chiefly distributed in Lijin subsag, and the overpressure of Es₃^x is inNiuzhuang and Lijin subsag. While, the overpressures in Boxing and Minfeng subsag arecomparatively weak. With the three periods hydrocarbon charging occurred, the formation fluidpressure gradually increases from the normal pressure to strong overpressure. In Niuzhuangsubsag, reservoirs which is above 2200m in depth was charged in the normal pressure andoverpressure formation system, while below 2200m was charged only in the overpressureformation system; in Lijin subsag, reservoirs which is above 2400m in depth was charged in thenormal pressure and overpressure formation system, while below 2400m was charged only in theoverpressure formation system.In Niuzhuang subsag, the upper parts of the Es₃^z sand bodies are acquifer, and the lowerparts are petroliferous. There are two oil accumulation models in the sand lens of Es₃^z. One,named outside source rock overpressrue compartment charge (OSROCC), is that oil and gastransformationally migrated from source rock to sands by subtle passage system, such assub-sequence faults and fissures; the other, named inside source rock overpreesure compartmentcharge (ISROCC), is that oil and gas directly migrated from source rock to sands. The oilsaturation of OSROCC sands is commonly higher than that of ISROCC sands. The porosity andpermeability of ISROCC sands is the main controlling factor of oil saturation. The oil potential inthe OSROCC sands decreases from bottom to top. Corresponding to the oil potential, the oilsaturation of the OSROCC sands generally decreases from bottom to top. Thus, we figured thatthe fluid potential plays an important role in the hydrocarbon accumulation of the OSROCCsands. The difference of the oil saturation of OSROCC sands is not obvious among the sandswhich have different physical property. The oil saturation is inapparently influenced by thephysical property of sands in the OSROCC.In Lijin subsag, the upper parts of the Es₃^z sand bodies are acquifers, and the lower parts arepetroliferous. The sub-fault is the chiefoil migration pathway in the lower of the section east, andin the upper parts, the visible fault is the chief oil migration pathway. The buoyancy, without theinfluence of the subtle passage system, drives the hydrocarbon from bottom to top in one singlesand lens. Accordingly, the upper of the lenticular sand body commonly has higher oil saturation.Physical property of sand bodies is the dominant controlling factor of oil lateral migration alongthe sand body updip direction.Only the third phase of hydrocarbon charging was measured in Boxing subsag. The oilmigration and accumulation is dominantly controlled by the fault and sand transportationpathways. Nearby the top of Es₄^s hydrocarbon migrated through the subtle passage system. Thefact that the reservoirs mainly distributed in the Es₃^z not the Es₃^x indicated that the high efficiency of the vertical transportation of faults. Beside, we found that were oil migrate informations of Es₃^z and Es₃^x with sub-faults and good properties of sands, both the fluid potentialdifference and the properties of sands controlled the oil migrating.