| Volcanic rock is quite different from sandstone and carbonate rock in diagenetic model, petrology and reservoir characterization.This study aims at enriching technologies for evaluation of volcanic reservoirs by means of the existing identifications of sandstone, carbonate rock facies and fractures.Volcanic reservoirs are closely related to fractures. Petrophysical properties of the reservoir tend to be reformed by fractures in or following diagenesis, which may provide channels for hydrocarbon migration at the stage of hydrocarbon accumulation; during the exploitation of the reservoir, they act as channels for oil-gas production and are of significant effects on the recovery process or model.There exist many factors affecting the fractures in volcanic rocks, such as tectonic movement, weathering denudation, subsurface fluid solution, rock mineral composition, and contents of various mineral constituents as well as inner structures in them, etc.This study, in view of Carboniferous volcanic reservoir in Shixi field of hinterland of Junggar basin, gathered a great quantity of data, including drilling, logging, seismic, conventional well log, imaging well log, cast thin section, well testing and production performance, and made lithologic discrimination, facies classification and fractural identification.The research of fractural identification was involved in fractural distribution pattern, fractural development analysis, fractural occurrence (high-mid-low dips) discrimination and fractural prediction modeling.Through these studies, preliminary results and understandings have been gained as follows:1) Propose a reverse-appraisal method that is more reliable based on imaging well log data to classify the lava flow orientations of volcanic rock facies, integrating with lava-debris thickness ratio.2) According to the fractures effected by tectonics, solution, weathering, rock mineral and inner rock structures, etc., understanding is given of fractural networks developed on the top of weathered crust in buried hill, and presented physical model for this networks, indicating that it is comprehensive actions by all kinds of factors that intensify fractal characteristics of the networks.3) Individual development of fracture is of its internal origin and law; while multi-developed fractures often occur in coincidence, forming existing non-order fractural system which results in complexity of appraisal of fractures.4) The major reservoirs are dominated by types of medium matrix-pores and developed fractures, with features of "large pores and small fissures" and "large fractures and micro pores". Statistic analysis shows that when matrix porosity is less than 5% or fractural density lower than 1 piece per meter, the volcanic rocks are difficult to become or shape reservoirs.5) By means of the ratio of deep and shallow resistivity as well as crossplot chart of GR, pay zone, aquifer and dry zone can be effectively identified in this area.6) FMI is used to visually identify fractures, but furthermore to calibrate conventional well log data and develop fracture prediction model.7) Fractures belong to a complex geologic body with non-linear effect on reservoir(s). Well log parameters vs fractural development is also non-linear, hence, multi-parameter evaluation is adopted as necessary. The individual parameter evaluation is effectively employed for local part rather than for whole area.8) The selection of sample parameters is significant for heterolithic samples have distinct features. Otherwise, the prediction model is poor in reliability.9) Using the prediction models for fractural occurrence and fracture-developed degree in Shixi volcanic reservoir developed in this paper has obtained prediction results in accordance with measured values up to over 75%.10) The research results can be used to determine fractural system parameters in repeat estimation of volcanic reservoir reserves, which possess an active significance for study of petroleu... |
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