| Fan-delta sandy conglomerates of Permian Xiazijie Formation to Triassic Baikouquan Formation are hosts of numerous stratigraphic - lithological hydrocarbon reservoirs, and have been exploration targets of Xinjiang Oil Company. Despite great hydrocarbon potential of these sandy conglomerates, a bottleneck problem in hydrocarbon exploration is to search for large-scale and effective reservoir rocks. Sandy conglomerate reservoirs, commonly observed in petroleum-bearing basins of China and characterized by intense heterogeneity and various pore types & structures, could be formed in complex settings. However, their formation mechanism, main controlling factors and geological prediction methods are poorly constrained. This study is, therefore, of great significance to deepening the understanding of sandy conglomerate reservoirs and their evaluation & prediction in China.This study, based on characterization of pore structures, is to investigate genetic types and formation mechanisms of pore structures in sandy conglomerates, with the ultimate purposes of understanding main controlling factors, constructing quantitative to semi- quantitative models for pore structure evolutions, and predicting the distribution of favorable sandy conglomerate reservoir rocks.According to above purposes, integrated analysis of sedimentology and diagenesis, microscopic diagenetic features and macroscopic diagenetic dynamic fields, and geological and experimental investigations are made in this study. The resultant understandings are listed below.1. Facies, distributions, and deposition mechanism of Xiazijie, Wuerhe, and Baikouquan formations are well constrained. Sediment sources, including Hongshanzui, Huangyangquan and Xiazijie, are of inheritance during middle Permian to early Triassic, when fan-delta and lacustrine facies were dominant in the study area. Sandy conglomerates in the study area can be classified into three types according to their textures, which are corresponding to two hydrodynamic depositional mechanisms, i.e. tractive, gravity currents. Argillaceous matrix contents in sandy conglomerates deposited through the above mechanisms are below 6% (here termed as net sandy conglomerate), and above 6% (here termed as graywacke), and they have great impact on pore structure evolution.2. Sandy conglomerates are of low compositional and textural maturity, and enriched in volcanic debris that are prone to hydration and hydrolysis. Vertically, compositional maturity of sandy conglomerates exhibits an increasing-upwardtrend trend, i.e. the content of quartz and feldspar in Baikouquan formation is obviously higher than those in Xiazijie and Baikouquan formations. Higher content of rigid grains is in some degree restraint to compaction, and thus favorable for porosity preservation.3. Reservoir space in sandy conglomerates consists of primary pores, dissolution pores and fractures. Dissolution pores are dominantly resulted from dissolution of volcanic debris, feldspar and less significantly, zeolite. Fractures could be generally classified into grain-penetrating and intra-grain ones, nevertheless, some micro-fractures resulting from clay mineral transformation and dehydration can be observed. Ratios of primary pores are higher in shallow-burial Baikouquan (48%) and upper Wuerhe (43%) formations than those in lower Wuerhe and Xiazijie formation(～25%). Sandy conglomerates are mostly of low to extremely low porosity and permeability, with porosity of 6% to 13% and permeability of 0.1 md to 30md. Sandy conglomerates of subaqueous distributary channel in fan delta front are potential high quality reservoirs.4. Pore structures in sandy conglomerates are grouped into high, medium, bad, and non quality ones, with the corresponding pore throat radium and permeability decreasing. Commercial oil flow can be acquired from sandy conglomerates with the first 3 categories of pore structures, and output of oil is exponentially increased with increasing pore throat radium and permeability.5. Pore structures of sandy conglomerates show close relationships with contents of matrix, detrital components, cements and compaction. Quality of pore structures gradually gets lower with increasing mud matrix.1) Sandy conglomerates with high and medium quality pore structures are deposited in middle to upper part of distributary channel in fan delta, and are poor in mud matrix, in which the amount of volcanic debris, compaction and cements are below 90%, 20% and 5%, respectively, with reservoir space of mostly primary pores and dissolution pores.2) Sandy conglomerates with bad quality pore structures are deposited in fan delta plain and front, which are mud-bearing, with the amount of volcanic debris, compaction and cements above 90%, ranging from 20% to 27%, and between 5% and 10%, respectively. Reservoir space in these rocks consists of primary pores, dissolution pores and micro-pores.3) Sandy conglomerates with non quality pore structures can be classified into mud-rich and mud-poor ones. The former is controlled by sedimentology, with cements below 3% and compaction above 27%, and reservoir space of intergranular and intercrystalline micro-pores; the latter is controlled by diagenesis, with cements above 10% or compaction above 27%, and reservoir space of intergranular and intercrystalline micro-pores.6. Three types of 3D pore throat structure numerical models are constructed by means of high resolution miro-CT.1) high connectivity pore volume model:composed mostly of primary pores, and sometimes of dissolution- primary pores;2) medium connectivity pore volume model:composed mostly of dissolution pores, and sometimes of primary - dissolution pores;3) low connectivity pore volume model:composed mostly of dissolution and micro-pores;4) extremely low connectivity pore volume model: composed mostly of dissolution and micro-pores.7. Sandy conglomerates have been experienced complex diagenetic events, including mechanical compaction by overlying load and tectonic compression, dissolution by acid, alkaline, and meteoric waters. Alkaline water dissolution played a crucial role in secondary pore development, while dissolution by meteoric and/or acid waters was locally occurred. Dissolution fluid in Lower Wuerhe formation and the underlying strata was commonly alkaline water; however, meteoric and acid waters were involved during early-middle and late diagenesis, respectively. Dissolution fluid in Upper Wuerhe formation was alkaline water, with fluxes of meteoric and acid waters in early and late diagenesis, respectively. On the contrary, dissolution fluid in Baikouquan formation was mainly acid water, while alkaline water was active locally.8. Sedimentology, heat flow, fluid activity and tectonic deformation are factors controlling diagenesis and pore structure evolution, and the basis for diagenetic type classification and formation mechanism investigation of pore structures. Pore structures in sandy conglomerates can be divided into 3 types (i.e. preserved one in net sandy conglomerate, altered one in sandy conglomerate, and syngenetic one in graywacke) and can be further subdivided into 6 types (i.e. thermally preserved one in net sandy conglomerate, tectonically-thermally preserved one in net sandy conglomerate, cementation- preserved one in net sandy conglomerate, dissolution-altered one in net sandy conglomerate, tectonic-altered one in net sandy conglomerate, and syngenetic one in graywacke). By analyzing the effect of each factor to pore structure evolution, quantitative to semi-quantitative models have been constructed.1) Preserved pore structure was mainly controlled by thermal maturity, lateral compressive stress and cementation that those sandy conglomerates experienced, which were responsible for the reduction of primary porosity.(1) Thermally preserved pore structure: The degree of primary porosity preservation was decreasing with increasing thermal diagenesis. This kind of pore structure was occurred in areas or intervals that experienced mild tectonics and dissolution. Evolution of this pore structure could be classified into 3 stages: ① when TTI (Time-Temperature Index) was below 8 (corresponding burial depth <2000m), pore volume and the quality of pore structure decreased rapidly with increasing TTI, with the decreasing rate of porosity averaging 1.6%/TTI and porosity/permeability ratio decreasing from 100md/% to 27md/%;② when TTI was in the range of 8 to 100 (corresponding burial depth of 2000m to 3500m), pore volume decreased slowly with increasing TTI, with the decreasing rate of porosity averaging 0.1%/TTI and the porosity/permeability ratio decreasing from 27md/% to 0.02md/%; ③when TTI was in the range of 100 to 400 (corresponding burial depth of 3500m to 4200m), pore volume decreased slowly with increasing TTI, with the decreasing rate of porosity averaging 0.071%/TTI and the porosity/permeability ratio decreasing from 0.02md/% to 0.003md/%. When TTI was above 400 (corresponding burial depth >4200m), the porosity/permeability ratio was below 0.003md/%, implying non reservoir quality.(2) tectonically- thermally preserved pore structure: imposition of tectonic diagenesis to thermal diagenesis accelerated reduction of pore volume and quality of pore structure. Evolution of this pore structure could be classified into 3 stages: ① when TTI was below 5 (corresponding burial depth< 1600m), decreasing rate of porosity was averaged 2.6%/TTI and porosity/permeability ratio was decreased to 0.17md/%;② when TTI was in the range of 5 to 10 (corresponding burial depth of 1600m to 2200m), decreasing rate of porosity was averaged 1.1%/TTI and porosity/permeability ratio was decreased to 0.06md/%; ③when TTI was above 10 (corresponding burial depth >2200m), decreasing rate of porosity was averaged 0.03%/TTI and porosity/permeability ratio was decreased to 0.003md/%. When TTI was above 200 (corresponding burial depth of 3700m), porosity/permeability ratio was below 0.003md/%, implying non reservoir quality.(3) cementation- preserved pore structure:Quality of this pore structure was dominated by the degree of cementation. Evolution of this pore structure could be generally classified into 2 stages, i.e. rapid decreasing of pore volume and quality of pore structure during early diagenesis, and slow decreasing of pore volume and quality of pore structure during late diagenesis. In mild tectonic areas, TTI of 2 to 3,11 to 23, and above 23 were related to medium, bad, and non reservoir quality, respectively. While in areas with intense tectonic compression (stress of-75MPa), TTI of 1.5 to 2,2 to 6, and above 6 were related to medium, bad, and non reservoir quality, respectively.2) altered pore structure in sandy conglomerate was that improved by dissolution or lateral compressive stress, on the basis of thermal diagenesis.(1) dissolution-altered pore structure:Pore volume and pore structure would be improved because of dissolution during diagenesis. This pore structure was occurred in areas or intervals that experienced fluid activities. Sandy conglomerates that experienced three episodes of dissolution showed medium, bad, and non reservoir quality when TTI was 100,120, and 500, respectively; while sandy conglomerates that experienced two episodes of dissolution showed medium, bad, and non reservoir quality when TTI was 30,100, and 500, respectively.(2) tectonic-altered pore structure:tectonically derived fractures can enhance reservoir quality especially permeability. This kind of pore structure was mainly distributed in areas or intervals that experienced intense tectonics. At the same interval, permeability of sandy conglomerates with tectonically derived intragranular fractures was 20 times higher than that without tectonically derived intragranular fractures.3) syngenetic pore structure was distributed in graywackes deposited in gravity currents. Because of no depositional porosity, this pore structure was of non reservoir quality since early diagenesis.9. The genetic model of sandy conglomerate reservoirs has been summarized. Difference in diagenesis and reservoir features is distinguished in the faulting zone, slope zone, sag zone, and forebulge zone. Sandy conglomerates without mud matrix were basis for the development of favorable reservoirs. Accordingly, developing regulations of sandy conglomerate reservoirs were analyzed and areas for favorable reservoirs were predicted. |
PDF Full Text Request