Study On Logging Evaluation Of Volcanic Reservoirs In Eastern Sag Of Liaohe Basin

Volcanic rocks were broadly distributed in Liaohe basin. Through years of exploration, many volcanic rocks with abundance reserves have been discovered. By the end of 2002, submitted proved oil in place in such reservoirs was 3169×104t. In Huangshatuo oilfield of eastern sag, established annual capacity is 25 ×104t, showing certain exploration and development future. Therefore, try to discover volcanic reservoirs and find new reserve replacement area have become an important orientation of exploration.Volcanic reservoirs belong to special lithology, fractured-porous reservoirs, and the reservoir evaluation is a difficult subject at home and abroad. Although experts have done much effort, solved a lot of problems, had certain experience, and reached agreement in some issues, they still have different opinions in geological understanding, exploration and development thought, and technical methods due to reservoir complexity, various geological conditions and technical level. According to geological data of volcanic rocks in the eastern sag of Liaohe basin, by learning and referencing advanced technical methods and experience at home and abroad, the study of logging evaluation methods has been conducted in six aspects, including identification of volcanic lithology, reservoir identification, reservoir classification, division of stages of massive volcanics, and reservoir parameter determination. Suitable methods of volcanic reservoir logging evaluation for this area have been summarized.Lithology identification: Because there are various minerals in volcanic rocks, i.e. the lothology is complicated, starting with petrophysical study like rock mineral analysis, through deep analysis of logging evaluation mechanism, four methods have been defined to identify lithology of volcanics, which are relation of K2O content and GR, M-N crossplot, ΦCNL and Rt crossplot, and ΦCNL, ρb, Δt and GR crossplot. Particularly, according to the relation of K2O content and GR, it is established to quantitatively identify basalt and trachyte by gamma ray curve. This method is precise and effective. ΦCNL and Rt crossplot is firstly used to qualitatively classify the alteration degree of volcanic rocks, i.e. unaltered or slightly altered Class I, medium altered Class II, strongly altered Class III, and pelitic Class IV. Reservoirs developed in volcanic rocks of Class II and III alteration, thus providing basis for reservoir evaluation.Reservoir identification: On the basis of accurate division of lithology, it is determined that trachyte is an effective reservoir rock. Based on analysis of the relation between oil property and physical property of core and production test data, the corresponding relation of logging information and reservoir characteristics is established. It is determined that quantitatively and qualitatively identify reservoir by Rt- Δt,ρh- Δ t and Rt-Φ CNL logging crossplot. Combining with conventional logging and core data, factors affecting image logging are studied and analyzed, and the mode of identifying true and false fractures by acoustic and electric image logging has been established. Conformance rate of reservoir interpretation has been improved by combining quantitative and qualitative identification by 3700 logging series and with the assistant of visual reservoir image logging.Determination of reservoir parameters: By analyzing mineral composition and using logging crossplot technique, Δ t, ρ b and Φ CNL value of volcanic matrix has been determined, and it is determined to calculatematrix porosity by A t. Through theoretical study, processing interpretation and application, it has been established to calculate fracture porosity by acoustic and electric image logging, thus solved the problem of porosity determining in reserve calculation.Reservoir classification and evaluation: According to classification of core capillary curves and data of production test, it has been first established to classify volcanic reservoirs by R,- A t crossplot. This method combines acoustic and electri...