Cause Analysis Of The Densification Of Ultra-low Permeability Asphalt Sandstone Reservoir: An Example From The Kepingtage Formation In Silurian Shuntuoguole Low-upwelling Area, Tarim

Generally searching for hydrocarbon enrichment zone with commercial value inlow-permeability tight sandstone reservoir depends to the prediction of the "sweetspot" reservoir. The foundation of "sweet spot" reservoir prediction is recognizing thecauses and process of the densification of sandstone reservoir precisely. Thecharacteristics of Paleozoic clastic rocks in Tarim basin has very low porosity andpermeability, and the exploration result has confirmed that asphaltic sandstones inKepingtage Formation has excellent exploration potential, thus asphaltic sandstones inKepingtage Formation has become a new area of oil-gas relay strategy for oil-gasexploration with " based on expanded resources, prospecting more resources andstabled oil-gas production " in Paleozoic clastic rock reservoir in Tarim basin. Studyon genetic mechanism of tight reservoir will contribute to understanding forpredicting forming mechanism and distribution of "sweet spot" reservoir precisely,and it is very important to effective exploration for the next step.Based on the exploration result of "The7thFive Years Programs for Science andTechnology Development of China" on new theory oriented of basin analysis,reservoir sedimentology, reservoir geology and hydrocarbon accumulation, analysison mineralogical characteristics and physical properties of reservoir rocks is startingfrom the core and logging data, through a large number of measured physical propertydata (with physical property of overburden pressure) and thin section data,summarizing the relevance among the clastic composition of sand reservoir, clasticstructure, interstitial material content and physical property. Based on the awarenesson sheet identification and scanning electron microscopic, the reservoir’s mainreservoir space and the characteristics of cement-matrix has been recognized;according to the observation on mercury injection data and image of cast thin sectionthe reservoir’s pore throat structure and pore distribution characteristics has beenaware of; and based on the data of homogenization temperature of fluid inclusions,biomarker chromatogram-mass spectrum test of asphalt extracts and thin section with fluorescent microscope, carbonate cement formation time with hydrocarbon chargingstage and asphalt occurrence has been recognized.Through the study on connection analysis of diagenetic evolution and reservoirporosity evolution, combines with analysis of clay minerals X-ray diffraction analysis,the controlling effect about deposition and diagenetic on reservoir quality can besummarized. Based on the categorization of tight reservoir, analyzed different causesof the densification of reservoirs, could summarize four results about developingcontrol factor and distribution of "sweet spot" reservoir, and results as follow:⑴Asphalt sandstone interval in Shuntuoguole low-upwelling area is mainlydistributed in the bottom of S1k3-3, and in the top S1k1, S1k3-1. It is formed by ancientreservoir which formed by early hydrocarbon charging suffered from leachingoxidation of meteoric water and biodegradation. Formed asphalted oil is mature andhigh-mature oil from source rock of Cambrian. Charging asphalt changes porestructure, destroys the reservoir quality. Membrane-asphalt in reservoir with finephysical property can prevent cementation, and it has a certain constructivesignificance. However, in reservoir with poor physical property membrane-asphalt forpreventing the water-rock reaction, is disadvantaged for corrosion reformation, and isdestructive.⑵Reservoir with poor physical property is typical ultra-low permeability tightsandstone reservoir. Mercury-injection curve has four forms. Corresponding porestructure has middle-small pore fine throat form and micro pore micro throat form,the developmental corresponding4types of reservoir, the fine reservoir of middlepore fine throat from is sweet spot reservoir. General reservoir is tight reservoirformed by compaction of high content cuttings. Non-reservoir which structure ismicro pore micro throat, is ultra-low permeability tight reservoir formed by a lot ofcarbonate cement and illite changing pore structure.⑶Compaction is the most important factor of pore structure damage. A lot ofcarbonate cement and the development of authigenic quartz cause lethal destruction ofreservoir quality. Common developmental illite causes the reservoir permeabilitydeclined dramatically; rigid particle content and coarse granularity is the key toporosity evolution of S1k1interval, a lot of primary porosities are saved inquartzarenite of S1k3interval, which is associated with relatively high content ofvolcanic lithoclast. Montmorillonite of alteration of volcanic lithoclast can formintegrated chlorite film in the quartz grain surface, and stop quartz overgrowth.However, because of the dissolution of potash feldspar and the development of illite and authigenic quartz in quartzose arkose, it has the coupling relationship of time andspace, multiple hydrocarbon filling is advantageous to the formation of secondaryporosity.⑷In the exploration of the S1k1interval is seeking a high content of primaryporosities area, the point is on the shore-shelf environment of high-energy facies belt.the point of the exploration of the S1k3interval is the floor sections with developedsecondary porosity. The next exploratory direction should in tidal-flat, deltaenvironment channel and sand body of the distributary channel underwater seek"sweet spot" reservoir.Research results have important guiding significance not only for reasonableexploration and deployment of Paleozoic clastic rock reservoir in Shuntuoguolelow-upwelling area, Tarim Basin, for promotion of exploration and developmentprocess, for realization of leading breakthrough of the tight sandstone field, but alsohave great realistic significance for realization of breakthrough in oil-gas explorationof Paleozoic deep clastic rock reservoir in the Tarim Basin. At the same time, it hastheoretical significance for deep-buried low-permeability sandstone reservoir,especially in asphalt sandstone reservoir seek and predict "sweet spot" reservoir,furthermore, it provides research value for the enrichment of petroleum geology andreservoir sedimentology theory.