Reservoir Characteristics And Petroleum Generation Potential Of The Late Paleozoic Volcanic Successions In Chepaizi Area Of Western Junggar Basin

Since the commercial oil flow was obtained in Carboniferous volcanic reservoir by P61 well in Shengli oilfield, by 2014, Carboniferous volcanic reservoir control 2627 × 104 t oil reserve, which indicate a huge exploration potential in the Carboniferous volcanic reservoir. With the further exploration, affected by Hercynian, indosinian, Yanshan and Himalayan movements, the diagenesis of work area were complex, some geological problems had not been solved, such as reserving space types and their combination characteristics and oil-bearing grad of different lithology and lithofacies were different, oil-bearing controlling factors were not clear. Clearing the reservoir characteristics and controlling factors as well as the relationship with the oil accumulation become the key of volcanic reservoir exploration in this area.On the basis of the previous research and the existing exploration results, this paper investigated the characteristics of volcanic lithology and lithofacies by the observation of cores and outcrops,and thin section analysis. The characteristics of volcanic reservoir and their controlling facters are studied by analyzing casting thin section, Scanning Electron Microscope and energy spectrum analysis, imaging logging data, as well as petrophysical data. The relationships between reservoir characteristics and oil accumulation are discussed on the basis of petrophysical data, casting thin section, geologic well log, hydrocarbon show by mud-logging, log interpretation and oil test data.Seven categories and 19 types are classified for the volcanogenic successions. The lithology are mainly basaltic rocks and sedimentary pyroclastic rocks. The andesitic volcanics and rhyolitic volcanic rocks account for a certain proportion. Five lithofacies and 13 subfacies are met. The main lithofacies is extrusive facies, followed by explosive facies and volcanogenic sedimentary facies, and volcanic conduit account for the lowest percentage. The reservoir space can be divided into four categories of original interstice, primary fracture, secondary interstice and secondary fracture, which include 13 kinds of space types. There are four types of effective reservoir space combination, varied by lithology and lithofacies. According to petrophysical data, the porosity of volcanic reservoir is medium to relatively high type, and the permeability of volcanic reservoir is low to ultra-low type. The influential factors of effective reservoir mainly include lithology and lithofacies, weathering and leaching, hydrothermal alteration, cementation and filling, and tectonism. Additionally, lithology and lithofacies affect reservoir distribution; weathering and leaching, hydrothermal alteration and tectonism contribute to reservoir pores, however, cementation and filling have destructive effect.Oil-bearing possibility of reservoirs is affected by lithology and lithofacies directly. Oil reservoir and poor oil reservoir are mainly distributed in air-fall, pyroclastic flow deposit and lower effusive facies, which include basaltic fragments lava, basalt, basaltic pyroclastic rock, Rhyolitic pyroclastic rock and polymictic pyroclastic rock. However, sedimentary pyroclastic rocks and subvolcanic rocks tend to form dry layer. The factors such as reservoir physical property, weathering leaching, hydrothermal filling and cementation and thick mudstone distribution have an important impact on oil-bearing possibility of reservoir. Additionally, oil-bearing possibility of reservoir has no relevance to reservoir physical property. Hydrothermal filling makes air hole and fracture useless and thus makes oil-bearing possibility of upper effusive facies become poor. Good oil-gas show has been obtained in weathering and leaching zone. Mudstone in the vertical direction plays a blocking role, causing the formation below itself uneffective.