Formation Mechanism, Reservoir Evaluation And Accumulation Of Deep Viscous Oil

Deep viscous crude oils,with depth more than 1800m,widely occur in Jiyang Depression,especially in Loujia-Kenxi region. Here,based on geological,geochemical and geophysical data,these oils were described and studied extensively in physical properties,sulfur contents,gross compositions,geochemical characteristics and reservoir features. Finally,we discussed the formation mechanism and outlined some exploration strategies of these viscous oils.The typical deep viscous oils in Loujia-Kenxi area are characterized by a high density (or low API gravity),a high viscosity,a high sulfur content and low amounts of saturates and aromatics as compared to resins and asphaltenes. The physical properties of these crude oils are closely related to their sulfur contents and gross compositions,hi addition to viscous oils,there are also some normal oils and intermediate oils between the normal and viscous types. As a result,the physical property,sulfur content and bulk composition of crude oils in this area vary significantly.Based on GC and GC-MS analyses,the geochemical characteristics of these typical deep viscous oils are listed as follows:(1) strong preference of even n-alkanes over odd n-alkanes;(2) strong preference of phytane over pristane,i.e. Pr/Ph ratio1;(3) high abundance of gammacerane relative to Caohopanes;(4) much more amounts of C35 pentakishomohopanes than C34 tetrakishomohopanes and even C33 trishomohopanes. Oil source correlation results demonstrate that these deep viscous oils are derived from the Es4 evaporite source rocks.Most of these deep viscous oils in Loujia-Kenxi region show no sign of biodegradation effects based on n-alkane and acyclic isoprenoid distributions. It appears that biodegradation,oxidation and water washing near the surface are not responsible for the high density and high viscosity of these deep viscous heavy oils,In addition,biomarker parameters demonstrate maturities of these viscous oils are relatively low to that of the normal oils derived from the Es3 source rocks in this area. It is possible that these deep viscous oils were generated from the Es4 evaporite source rocks during the early oil generation stage. After further burial,their maturity parameters increased while their physical properties remained unchanged.hi addition to these native viscous oils,there are also typical altered viscous ones in Jiyang Depression,of which the gas chromatograms show clearly the features of biodegradation effects. The biomarker characteristics of these two types of viscous oils are distinctive. Oil source correlation results show the altered viscous oils were generated from the Es3 source rocks and subsequently suffered biodegradation,oxidation and water washing effects near the surface.There exists huge thick alluvial glutinite in Luojia-Kenxi area,with various physical properties. By applying the comprehensive study idea of geostatistical analysis and geophysics,the reliable reservoir distribution is described in the complicated thick glutinite body. By comprehensive analyzing log multi-parameters,oil,gas and dry layer are identified,and the reservoir distribution in Luojia-Kenxi area is further described. The reservoir and hydrocarbon prediction are reliable provedby the drilling of production preparing wells.On the basis of systematic research,it is shown that there exist great resource potential and different exploration targets in deep viscous crude of Jiyang depression.