Abstract:

Based on the systematical research and statistical analysis on lithologic oil reserviors in Jiyang depression, it is found that the formation of lithologic oil reservior is mainly controlled by hydrocarbon resource, reservoir petrophysical property and sandbody genetic type.In the geological history, the effective hydrocarbon source rocks were subjected to increasing temperature and pressure during burial period.When the isolated sandstone body contacts with or is surrounded by effective source rock and only when the hydrocarbon source rock reaches hydrocarbon generation and expulsion threshold during burial to greater depths, migration and accumulation of petroleum can take place in significant quantities.When those sandstone does not contact with effective hydrocarbon source rock, migration and accumulation of petroleum can also take place if faults link up the lower, upper or deep source rocks. Although the lithological oil reservoir can be developed in various sedimentary environment (channel,delta,cestal bar and turbidite fan), the sandbodies developed in different sedimentary environment have different hydrocarbon-potential.The areal distribution and hydrocarbon-potential of lithologic sandbody are controlled by deposition. The lithologic sandbody is best developed in turbidite fan and has better hydrocarbon-potential, the delta and subsea-fan sandstones have secondary position in hydrocarbon-potential. Even in turbidite sandbody, the deep-water turbidite is different from delta front fluxoturbidite in hydrocarbon-potential. The hydrocarbon filling factor in deep-water turbidite is higher than that in delta front fluxoturbidite.The porosity and permeability in sandstone are also major factors that affect and control the hydrocarbon filling factor in sandstone. Sandstone cann't be filled with oil and gas from hydrocarbon source rock until the porosity and permeability in sandstone reach their critical values.From analysis of major factors that control the formation lithologic oil reservoirs, it is found that the hydrocarbon source rock cann't generate enough hydrocarbon for oil and gas accumulation in trap until the source rocks are buried to depth threshold of hydrocarbon generation.The sedimentary environment of trap formation affects hydrocarbon accumulation. Under the control of deposition, oil and gas can only be accumulated in some types of traps.Moreover,the internal condition of sandstone,i.e.petrophysical properties of sandstone, also control hydrocarbon accumulation in traps. Therefore, hydrocarbon accumulation in lithologic traps can be explained by three-genetic factors. In the ternary composite accumulation model,three end members are represented by sandstone type (i.e. sedimentary environment) buried depth and hydrocarbon-expelling intensity(i.e.hydrocarbon generation and expulsion), sandstone petrophysical property (i.e. internal condition of sandstone)respectively.The hydrocarbon generation and expulsion condition and sandstone internal-condition range from bad to good, sedimentary environment range from unfavorable to favorable. Trap hasn't hydrocarbon accumulation until all three-genetic factors satisfy certain conditions.On the basis of fine analysis on hydrocarbon source condition, sedimentary system, sedimentary facies and reservoir petrophysical properties for the sequence I in lower Es₃ in Chezhen depression, the ternary genetic mechanism was used to make preliminary and comprehensive prediction on the lithologic pools in sequence I in lower Es₃ in Chezhen depression. According to prediction results, we suggest that the important turbidite fan bodies in Chexi and Guojuzi sags are important exploration targets, where the Che 271 and Da 355 wellblocks are favorable regions for searching lithologic trap and lithologic pools.

Key words: lithologic oil reservoir, major controlling factors, petroleum accumulation process, three-genetic factors, Chezhen depression