Characterization Of Effective Gas Reservoirs Hosted In The Lower Cretaceous Volcanic Rocks Of Songliao Basin

In Songliao basin, eighty-seven percent of the proved gas reserves are stored in the Lower Cretaceous (K1y) volcanic rocks mostly below 3000 meters. To present, there are more than two hundred boreholes revealing volcanic rocks with proved gas reserves up to 321.8 billion cubic meters and under proved gas reserves up to 500 billion cubic meters, also showing potential reserves more than 1000 billion cubic meters. In Xujiaweizi (XJWZ) depression, the volcanic gas reservoir now is undergoing precise exploration phases; quantitative characterization and evaluation are needed for offering credible geological basis for borehole and artificial fracturing location optimization and reserve calculation.This paper made integrated descriptions on volcanic lithology, lithofacies, volcanic cycles and volcanic edifices with geological, logging and seismic methods. Reservoir types were divided by gas and water productivities and different type of reservoirs was characterized respectively, and summarized their characteristics, controlling factors and spatial distributions. On the basis of reservoir analyses and quantitative characterization, analyzed the formation mechanism of effective reservoirs and seek for theirs distinguishing sighs and controlling factors. Quantitatively analyzed and evaluated reservoir porosity, permeability, heterogeneity and pore structures. Based on reservoir categorization by gas and water productivities and clustering analyses, determined the lower limit of effective reservoirs as well as the boundaries and ranges for classification and evaluation.On the basis of this research, summarized a series of theories and methods for the characterization, classification, evaluation, identification and forecasting of volcanic gas reservoirs with low permeability, intense heterogeneity among multiple lithologies. 1. Characteristics of lithology, lithofacies, cycle and volcanic edifice of the Yingcheng formation, XJWZ fault depressionVolcanic rocks of the Yingcheng formation include more than 20 types ranging from mafic to felsic and from lava to volcanoclastic rocks. the lithological composition, structure and texture are identified with core, cuttings by polar microscope, major elements analysis,and electron probe.Based on the lithological analysis, the logging data is demarcated and the correlation between lithology and logging are established with crossplots. According to the lithology-logging identification model, the lithology of core absent part is interpreted. According to different features of FMI image of volcanioclastic rocks, matrix/skeleton, fracture, pore, combined with core demarcation, the logging image identification mode for rhyolite structure, massive structure, stomata - almond structure, welded tuff structure,volcanic breccia structure and crystal tuff structure are established.There are three types of volcanic facies emplacement in the Yingcheng formation including filling,drape and mound-building. The 1st member and the 3rd member of Yingcheng formation are divided into four and three volcanic eruption cycles, respectively. In general, there are tendencies of mafic-intermediate-felsic (magma composition) for lithology and effusive facies-explosive facies- extrusive facies (volcanism ) for lithofacies.On the basis of quantitative analysis of outcrop and buried volcanic edifice, according to lithological composition, facies sequences and facies composition, There are six types of buried volcanic edifices facies modes established for acidic lava volcanic edifice, intermediate pyroclastic volcanic edifice, acid composite volcanic edifice, intermediate lava volcanic edifice, intermediate pyroclastic c volcanic edifice and intermediate-mafic composite volcanic edifice, respectively.2. Reservoir spaces and diagenesisVolcanic reservoir spaces are mainly composed of holes,caves and fractures. According to the diagenesis and reformation, the volcanic reservoirs can be classified as two genetic types including primary and secondary ones, and 11 types and 20 subtypes can be compartmentalized further according to the genetic mechanism, shape and distribution characteristics. Primary reservoir space means various open apertures which formed in closed system conditions before the complete cooling of volcanic rocks and in primary diagenesis condition. The primary reservoir spaces include five types as stomatas, framework holes, contraction cavity, shrinkage fracture and shattered fracture. While secondary reservoir spaces formed from the alteration of primary reservoir types or generating new reservoir types on the condition of opened system after the complete cooling of volcanic rocks and controlled by secondary diagenesis. The secondary reservoir spaces mainly composed of dissolution pores, hydrothermal fractures, weathering fractures, dissolution fractures, pressure solution fractures and structural seams.The stage of diagenesis of volcanic rocks can be divided into early diagenesis and late diagenesis. Early diagenesis mainly controls the formation and distribution of primary reservoir spaces and consists of volatile component overflow, solidification contraction, welding and autoclastic brecciation and so on. The late diagenesis mainly influences the alteration of primary space and the development of secondary space. The late diagenesis includes compaction, hydrothermal brecciation, filling, dissolution, weathering and leaching and tectonic process.3 Quantitative evaluations of porosity, permeability and heterogeneityGas and water were reserved in and outputted from multiple lithologies ranging from mafic and felsic, lava and pyroclastic volcanic rocks in XJWZ depression. Among these diverse volcanic rocks, effective gas reservoirs are mainly developed in rhyolite, ignimbrite, trachyte and basalt.Effective porosity, gas permeability, density and pore throat were used to analyze and evaluate reservoir properties and heterogeneities of volcanic reservoirs. Proved that porosity from high to low are sequenced as rhyolite, ignimbrite, trachyte and basalt while the last had the highest permeability.Heterogeneity of volcanic reservoirs display intra and between layers and in plane. Intralayer heterogeneities behaves as porosity and permeability diversities vertically in separate cooling units, proved that permeability differ greater than porosity and vertical permeability differ greater than horizontal. Intralayer heterogeneity of different lithologies from high to low are sequenced as basalt, ignimbrite, rhyolite and trachyte which is opposite to the difference of reservoir effective thickness between them. Heterogeneity between layers is resulted to diversities of porosity and permeability between different volcanic cycles, periods and cooling units. Heterogeneity in plain is related to differences of porosity and permeability between separate volcanic bodies or among different facies.4 Quantitative characterizations of pore structures Quantitative analysis and evaluation of pore structure was made for the volcanic rocks on the basis of regular mercury testing. Pore throat diameter of volcanic rocks ranges between 0.016μm and 10.64μm which corresponds to the dimensions from tight sandstone to sandstone (0.03~20μm). Generally, the maximum pore throat diameter of effective volcanic reservoirs is close to sandstone reservoir (2~20μm), and the pore diameter of non-effective volcanic reservoir coincide with tight sandstones (0.03~2μm).Pore throat radius of effective reservoir and non-effective reservoir differ obviously for basalt and ignimbrite tuff with greater heterogeneity, the gap is not obvious for trachyte and rhyolite with no great heterogeneity.The pore throat radius could be divided into five grades, sequenced as macro, coarse, medium, small and micro with boundaries as 0.63μm, 0.16μm, 0.063μm and 0.04μm. Based on the size of pore throat radius and their distribution histogram, summarized seven types of pore distributions including single coarse-kurtosis, single-bias coarse-kurtosis, single-bias fine-kurtosis, single fine kurtosis, double-bias coarse-kurtosis, double-bias fine-kurtosis, single micro-kurtosis. Effective reservoirs mainly develop single coarse-kurtosis, single-bias coarse-kurtosis and double-bias coarse-kurtosis, single-bias fine-kurtosis, single fine kurtosis, double-bias, fine-kurtosis correspond to both effective and non-effective reservoirs. And single fine-kurtosis is mostly in empty layers proved by daily productivity.5 Lower limits of volcanic effective reservoirs and reservoir classification and evaluationEffective reservoirs are those with daily productivities above industrial levels under present artificial techniques and economic benefits.Porosity, permeability, heterogeneity and pore throat distributions are different among rhyolite, ignimbrite, trachyte and basalt so that their lower limits and boundaries need to be determined separately.The lower limit values of porosity and permeability and determined by the distribution function curve method, cross-plots and statistical method. Average value of the lower limits determined by the distribution function curve method and corss-plots are considered to be the lower limits of effective reservoirs. While the statistical value is considered to be the upper limit of empty layers. These results coincide well with the values of porosity and permeability in boreholes with daily production data. Five categories of volcanic reservoirs are determined by daily productivity of gas and water, classⅠare those reservoirs with productivity greater than forty thousand cubic meters per day without artificial fracturing, classⅡ,Ⅲ,Ⅳ,Ⅴare sequenced with the boundaries as 10×104m3/d, 4×104m3/d and 100m3/d. ClassⅠ,Ⅱ,Ⅲare effective reservoirs, classⅣandⅤare non-effective ones. ClassⅠa ndⅡare mainly developed in rhyolite and ignimbrite. Basaltic reservoirs mostly are classⅢ,ⅣandⅤ. Trachytic resvoirs mostly are classⅡandⅢ.6 Distribution, identification and forecasting of volcanic effective reservoirsProved by lithologic, mineralogic and geochemical studies, albitization, hydrothermal brecciation and multiple component amygdules indicate effective reservoirs.Vesicular zone and middle welded zone are according to the location of effective reservoirs, which can be identified with high DT and low Density due to primary porosities. And the massive zone of lava and densely welded ignimbrite coincide with low DT and high Density due to little porosities.Separately establish pertinency between volcanic edifice and facies-belts, volcanic facies, seismic attributes and volcanic reservoirs. Superpose the evaluating and forecasting results determined by the three methods in order to reveal reservoir categories on the plane. Ten effective reservoir developing blocks are determined in the 3rd cycle of K1y1 volcanic rocks with an area of 360 km2, and non-effective reservoirs covers 460km2. Six effective reservoir developing blocks are determined in the strata of K1y3 intermediate and mafic rocks with an area of 229 km2, and non-effective reservoirs covers 53%.