Geological-Logging Interpretation And Evaluation Of Paleogene Basaltic Reservoirs In Liaohe Depression

Volcanic activities were intense and frequent in Paleogene of Liaohe depression,leading to layered and widespreading basalts developed in each strata vertically and among all the three sags horizontally,while the basalts are also close related to reservoir formation and hydrocarbon accumulation.Based on drilling,logging data and relevant reservoir analysis and test results of Paleogene basalt in Liaohe depression,this paper defines the lithology and lithofacies types of basalt,analyzes and summarizes the logging response characteristics and logging facies types of basalt.Through the analysis of effective porosity,diagenesis and reservoir physical properties,the development characteristics and distribution law of dominant reservoir are determined,and then the comprehensive interpretation and evaluation of dominant basalt reservoir are realized through the combination of geological and logging data.1 Lithology and lithofacies type of basaltThe lithologic types of Paleogene basalt include volcanic lava and pyroclastic rocks.Volcanic lavas are specifically divided into four categories: massive basalt,vesicularamygdaloidal basalt,brecciated basalt and cataclastic basalt.Pyroclastic rocks can be divided into three basic types: basaltic tuffs,basaltic breccias and basaltic agglomerates.Liaohe depression basalt develops three facies and seven subfacies,specifically volcanic conduit facies(volcanic neck and hydrothermal berccias subfacies),explosive facies(pyroclastic flow,air-fall subfacies)and effusive facies(composite flow,tabular flow and hyaloclastite subfacies).2 Logging response characteristics and logging facies analysis of basaltAnalysis and determination of deep lateral resistivity(RLLD),compensated neutron(CNL),density(DEN)and acoustic time difference(AC)are the sensitive curves for logging interpretation and evaluation of basalt reservoir.On the basis of logging curve correction,the logging response and logging facies characteristics of basalt are analyzed from five aspects: amplitude,shape,top bottom contact relationship,smoothness and logging parameter response range.The results show that:(1)Basaltic volcanic lava is divided into massive basalt and non-massive basalt by neutron-density crossplot,The development degree and vertical distribution of vesicular,breccia and fractures cause the regular changes of logging curve shape of basaltic volcanic lava.Combined with the curve shape,the non-massive basalt is further divided into vesicular-amygdaloidal basalt and brecciated-cataclastic basalt.vesicularamygdaloidal basalt is dominated by smooth or micro toothed curve,which is generally developed in the top and bottom of box curve and toothed curve.Brecciated-cataclastic basalt is mainly toothed curve.The basaltic pyroclastic rocks as a whole show low resistivity,medium to high neutron,with the increase of pyroclastic material content,the density gradually increases and the acoustic time difference gradually decreases;The degree of homogeneity and vertical variation of particle size and composition of pyroclastic materials are the key factors causing the regular changes of logging values and curve shape of pyroclastic rocks.(2)The corresponding relationship between the morphological characteristics and combination of logging curves and basalt subfacies is established.Tabular flow subfacies are mainly medium low amplitude box curves,mostly smooth or micro toothed;Composite flow subfacies are mainly micro toothed box curves or toothed and finger curves superimposed with each other.hyaloclastite subfacies are micro toothed smooth curves or medium low amplitude toothed curves;air-fall subfacies generally have low amplitude micro toothed smooth curves;pyroclastic flow are smooth curves or superposition of medium high amplitude tooth curve and finger curve.3 Comprehensive evaluation of dominant basalt reservoir.The dominant reservoir of Paleogene basalt in Liaohe depression are mainly developed in vesicular zone,fracture zone and alteration zone.According to the vesicular content,the vesicular basalt is divided into massive layer,less vesicular layer,medium vesicular layer and vesicular layer.Combined with the measured porosity calibration,the logging evaluation standard of basalt vesicular development degree is established.The resistivity coefficient is calibrated by fracture face ratio,and the logging evaluation standard of basalt fracture development zone is established.It is divided into three categories: under developed,relatively developed and developed.Based on the clay mineral content,basalt is divided into three levels: no alteration or slight alteration,moderate alteration and severe alteration.Combined with deep lateral resistivity and compensated neutron,the logging evaluation standard of basalt alteration degree is established.