Method for effective sandstone reservoir identification and evaluation - a case study from the clastic reservoir in Kuqa depression, Tarim Basin

Hydrocarbon exploration results show distribution of oil and water is complicated in clastic reservoir for many oil-bearing basin in China. Water accumulates in the high porosity, high permeability and shallow formations.... [ view full abstract ]

Hydrocarbon exploration results show distribution of oil and water is complicated in clastic reservoir for many oil-bearing basin in China. Water accumulates in the high porosity, high permeability and shallow formations. While in the deeper formations with low porosity, and low permeability, oil and gas reservoirs are formed. This indicates the identification and evaluation of effective reservoir for hydrocarbon accumulation cannot be determined by the physical properties of rocks alone. In this article, the effective reservoir identification and evaluation in the Kuqa depression, NW China, were investigated through laboratory physical simulation experiments and hydrocarbon migration dynamic analysis. We also use core physical property analysis, well-logging data, and production test data to determine the physical property threshold of effective reservoir. Finally, we predict the distribution of effective reservoir in the research area and provide an important foundation for further oil and gas exploration.
The results show the porosity and permeability of reservoirs and surrounding rocks generally decrease with increasing burial depth in the Kuqa depression. At given equal depth, hydrocarbons will only accumulate where the porosity and permeability are relatively high, and there is a reservoir physical property threshold for hydrocarbon accumulation. The porosity threshold decreases (from 22 % to 4 %) with the burial depth increasing (from 1800 m to 5400 m). In our experiment, oils always migrate from small particle size surrounding rocks with poor physical properties to large particle size reservoirs with better ones. When the surrounding rock capillary force was two times lower than that of reservoir, the hydrocarbon accumulation can’t occur in the reservoir, and the higher the ratio of surrounding rock capillary force to sandstone reservoir capillary force, the higher hydrocarbon accumulation probability of the reservoirs is. The ratio on the critical conditions become larger with increasing depth, which means the capillary force threshold ratio has a certain relationship with burial depth. Thus, the capillary force ratio (CPR) can be used to identify the effective reservoir. The higher the CPR, the better the reservoir quality is, and the more chance for hydrocarbon enriching in the reservoir. The CPR index was established for quantitative evaluation of reservoir quality in the Kuqa depression, and has obvious positive correlation with the reservoir oil (gas) saturation, which means the method has certain reliability. The new method predicted effective reservoirs for hydrocarbon accumulation mainly located in the south area of Kela2 Gas Field and north area of Dina gas field.