This paper introduces a concept of “lower boundary of buoyancy-controlled hydrocarbon accumulation”. The lower boundary refers to the dynamic boundary conditions of hydrocarbon accumulation under the buoyancy force，which coincides with the depth at which porosity is between 10% and 12%.Above the boundary, the buoyancy force dominates the hydrocarbon migration and accumulation.As a result, the conventional oil and gas reservoirs develop in the dynamic field.When it is deeper than the boundary, capillary force restrain the free migration of hydrocarbon,casuing them to accumulate in the tight reservoirs with sealing by hydrostatic pressure,which indicates tight hydrocarbon reservoirs develop in this field.Thus,the determination of the boundary has great significance for us to do oil and gas exploration.And sedimentology and diagenesis have many effects on the boundary through the research.
The “lower boundary of buoyancy-controlled accumulation” has been studied through the analysis of porosity evolution model which was based on the following formula. Eleven parameters were reviewed for their influence on porosity,which include depth,pressure,median grain size,sorting,reservoir age,cement contents,clay contents,feldspar contents and so on.Then the porosity relationship was investigated by using the step-wise multiple-regression program through the software “SPSS”.This program selects those parameters that contribute in a statistically significant way to the regression.289 groups data from 40 wells in the Dongpu Depression have been used,and a formula of porosity can be obtained from the program:
φ=181.766-19.709*X1-0.232*X2+3.422*X3-0.262*X4+6.774*X5-0.042*X6-0.538*X7+0.036*X8
R2=0.75
where φ represents porosity,X1 refers to the logarithmic depth of the reservoir,X2 denotes the content of cements,X3 is pressure coefficient of the reservoir,X4 designates ages of the reservoir rock,X5 is median grain size,X6 refers to the clay content,X7 represnts the content of feldspar,X8 is sorting coefficient of reservoir rocks,R represents correlation coefficient. The root-mean-square error between estimated and well-logging porosity is only 1.5% and the correlation coefficient is 0.985,whcihc indicate it is a reliable method.For the study area,the average X2,X3,X4,X5,X6,X7,X8 of the reservoir will be used,and the porosity of 12% is considered as the standard that corresponds to the“lower boundary of buoyancy-controlled accumulation”.In this case,the corresponding depth of the boundary is about 3580m in Dongpu Depression.
The study results show that the boundary is affected by sedimentology and diagenesis.To study the specific relationship between a certain factor and the boundary,the certain factor will be changed and others will keep unchanged.And the results show:grain size,sorting,pressure,feldspar contents have a positive relationship with the boundary,respectively;cement contents,clay contents,reservoir age have negative relationship with boundary, respectively.
Topics: Physical sedimentary processes , Topics: Diagenesis , Topics: Lacustrine sedimentation
