Genetic Mechanism And Evolution Model Of Secondary Pore Development Zone Of Paleogene In The North Zone In Dongying Depression

The genesis and evolution of secondary pore development zone is important basis for the high-quality reservoir evaluation and the deployment of oil and gas exploration in mid-deep formation. Generally, several secondary pore development zones are developed in a petroliferous basin. At present, it is considered that feldspar and carbonate cements dissolved by organic acids is the cause of the first secondary pore development zone corresponding to organic matter maturity in shallow formation. While,there are two views on the causes of deep secondary pore development zones. One is that deep secondary pore development zones were formed in deep formation;Another is that deep secondary pore development zones were the secondary pore development zones forming in the shallow formation and preserved to the deep formation. If the genesis and evolution of secondary pore development zones can't be determined, the success rate of hydrocarbon exploration will be affected a lot. So the research on the genetic mechanism and evolution model of secondary pore development zones has great leading significance in theory for oil and gas exploration in mid-deep formation. This dissertation focused on the case study of the sand-conglomerate body of the middle part of Es3z～Es4 of Paleogene in Dongying depression. Based on a conjunction of core observation, thin section identification, image analysis, SEM observation, fluid-inclusion analysis, core X-ray analysis, testing of vitrinite reflectance and core properties analysis and other methods, and the research achievements as the tectonic evolution history, thermal evolution of organic matters, and the history of hydrocarbon accumulation, the genetic mechanism and evolution model of secondary pore development zones in mid-deep formation were systematically studied.Secondary pore development zone is defined as the zone where high porosity reservoirs (also called effective reservoirs), whose secondary pore is more than 50%, intensively develops. Accordingly, the study shows that the secondary pore development zone in Es4 of Minfeng sag is most typical. It develops 4 secondary pore development zones totally. The secondary pore development zoneⅠandⅡdevelops in Es4s, the depth range of which is 2800m-3500m and 3750m-4050m. The secondary pore zoneⅢandⅣdevelops in Es4x, the depth range of which is 4300m-4500m and 4700m-4900m. So this dissertation focus on the discussion of the secondary pore development zones of Es4 in Minfeng sag.The reservoirs of the secondary pore development zones in Es4 in Minfeng sag are sand - conglomerate of braided channel in nearshore subaqueous fan, and the secondary pores of which are mainly feldspar dissolution pores, secondly carbonate cement dissolution pores, a few quartz dissolution pores, pyrite dissolution pores and cracks. The reservoirs of secondary pore zoneⅠandⅡare mainly oil reservoirs, and secondly a few oil-water layers and oil-bearing water layers. The reservoirs of secondary pore zoneⅢandⅣare gas layers. The typical diagenetic characteristics of reservoirs in secondary pore development zoneⅠ,Ⅱis that the feldspar dissolution is strong, the phenomenon that feldspar dissolution pores filled by carbonate cements is common, and carbonate cements dissolution is obvious. The typical diagenetic characteristics of reservoirs in secondary pore development zoneⅢandⅣis that a lot of pores were filled by asphalt and pyrite, carbonate cements are high-shaped and dissolved weakly, feldspars are also dissolved weakly, and the phenomenon that feldspar dissolution pores filled by carbonate cements is not obvious. The reservoirs in secondary pore development zoneⅠandⅡexperienced the diagenetic environment with"alkaline-acid-alkaline-acid-weak alkaline","normal pressure - normal pressure - normal pressure","formation temperature increasing - decreasing - increasing", and diagenetic evolution sequence of which is"compaction / pyrite cementation / siderite cementation / gypsum cementation→feldspar dissolution / quartz overgrowth→carbonate cementation/ quartz solution / feldspar overgrowth→carbonate and feldspar late period solution / pyrite cementation". The reservoirs in secondary pore development zoneⅢandⅣexperienced the diagenetic environment with"alkaline - acid -alkaline-acid-weak acid","normal pressure - overpressure-normal pressure","formation temperature increasing-decreasing-increasing", and diagenetic evolution sequence of which is"compaction / gypsum cementation / halite cementation / gypsum cementation / pyrite cementation / siderite cementation→feldspar dissolution / quartz overgrowth→carbonate cementation / quartz solution / feldspar overgrowth→carbonate dissolution / feldspar dissolution / quartz overgrowth→pyrite cementation and asphalt filling".Based on the research of the diagenetic environment evolution and the diagenetic sequence of the secondary pore development zones, the models of diagenetic environment evolution and reservoir reconstruction in Es4s and Es4x have been established respectively. The genetic mechanism of the secondary pore development zoneⅠandⅡis that the middle reservoirs of brained channels of mid-fan of nearshore subaqueous fan in middle-upper part of sedimentary cycle formed pore development zone with the primary pore in leading position because of massive secondary pore formed by early organic acid dissolution, and because of the slight destruction of primary pore by carbonate cementation, the pore development zone formed the paleo secondary pore development zone, which experiences the dissolution effect again by organic acid in the process of strata uplifting, and the paleo secondary pore development zone formed the present secondary pore development zone finally due to the protection of hydrocarbon charging. The genetic mechanism of the secondary pore development zoneⅠandⅡcan be summarized as"dissolution in acidic environments is main, dissolution in alkaline environment is secondary, secondary porosity formed in early stage, secondary porosity development zone formed in early stage".The evolution model he secondary pore development zoneⅠandⅡcan be summarized as"early generated-multiple phase deformation-late hydrocarbon charging protection". The genetic mechanism of the secondary pore development zoneⅢandⅣis that the middle reservoirs of brained channels of mid-fan of nearshore subaqueous fan in middle-upper part of sedimentary cycle formed the pore development zone with the primary pore in leading position (the absolute amount of secondary pore is high) because of the early organic acid effect, and because of early hydrocarbon charging and overpressure protection, the pore development zone was slightly transformed during the process of strata uplifting and resettlement, and the pore development zone finally formed the present secondary pore development zone due to the filling of primary pore by massive asphalt and pyrite from oil cracking of deep buried paleo-reservoirs (the buried depth was more than 4000m and the formation temperature was more than 160℃). The genetic mechanism of the secondary pore development zoneⅢandⅣcan be summarized as"dissolution in acidic environments is main, dissolution in alkaline environment is secondary, secondary porosity formed in early stage, secondary porosity development zone formed in early stage".The evolution model of the secondary pore development zoneⅢandⅣcan be summarized as"early hydrocarbon charging- overpressure protection-late cause".