| The clastic rock oil reservoirs of Yanchang Formation mainly developed in low permeability sandstone traps which are non-structural and composite traps, in Ordos Basin. However, detailed correlation of sandstone reservoirs, fine describes of sedimentary system and formation growth characteristics make it difficult for oil-gas exploration in this area. Due to the control of multi-sources, vertical and horizontal differences of reservoir characteristics and densification of low permeability, the key exploration factors of Yanchang Formation were affected by the development of dense sandstone reservoir and the relationship of densification process and petroleum accumulation. Based on the study of Chang8 oil layer group of Yanchang Formation in southwestern Ordos Basin, the isochronal stratigraphic framework with Chang8 character is established. The construction of isochronal stratigraphic framework help to reveal the constitution, spatial distribution and its sedimentary evolution features of sedimentary microfacies. We also build a prediction model to study fine reservoir body of Chang8 tight sandstone. Combined with analysis of reservoir characteristics, this study try to carry out the controlling factors of tight sandstone reservoir development and its densification mechanism in order to provide abundant geological evidences for describing and predicting of low permeability tight sandstone’s fine reservoir body. The results are as follow.(1) The stratigraphic framework of Chang8 oil layer group of Yanchang Formation, southern Ordos Basin, is established. Gentle slope sequence framework with foreland-like type of flexural basin is carried out. Based on a comprehensive research method, such as outcrop observation, core observation, geological prospecting, seismic exploration and logs,2 first-order sequence boundaries (bottom boundary and top boundary of Yanchang Formation),1 second-order sequence boundary (bottom boundary Chang6 oil layer group) and 2 third-order sequences boundaries (bottom boundary Chang8 oil layer group, bottom boundary Chang3 oil layer group) are identified in Yanchang Formation. The 2 first-order sequence boundaries are paleo-structure surfaces of basin level, which means regional unconformities. The 1 second-order sequence boundary is structure surface of depression level, which represents the stress transition of tectonic stages. The 2 third-order sequences boundaries are regional tectonic surfaces, which is facies transition surface. Accordingly, Yanchang Formation is divided into 4 third-order sequences. The formation of sequence is mainly controlled by episodic tectonic activities. Chang8 oil layer group is mainly corresponding to lowstand system tracts of Chang8-Chang7. Due to the recognition of several lakes flooding surfaces in Chang8 oil layer group, which can be further divided into 6 parasequence formations and 15 parasequences. Generally, Chang8 oil layer group can be divided into two evolution cycles from bottom to top, which are Chang82 and Chang81. The strata distribution inside the stratigraphic framework has not only stable thickness but also easily to be compared with. And the strata distribution was with the characteristics of shallow water and gentle ramp during its developing period. Thus, inside the stratigraphic framework, the sedimentary and filling of the strata is vertical deposition with no obvious sedimentary environment changes.(2) Comprehensive utilization of analysis of structure background of clastic rock provenance, slight mineral combination, cathode luminescence feature and REE, the provenance characteristics and direction in study area is analyzed. It is assumed that the provenance is originating the southwest margin of Longxi archicontinent where developed middle-low-grade metamorphic rocks and acidic magamtic rocks.The sandstone group of Chang8 oil layer group distributed in recycled orogeny, cutting arc and transition arc in diagram for Dickinson. According to the irradiation mechanism of cathode luminescence, quartz color is mainly brown and dark brown, sometimes is purple. Potassium feldspar color is mainly blue, plagioclase is green and albite is red. Feldspar has a strong weathering. Calcite color is bright orange yellow and calcite is replaced and cemented. Bit red dolomite cements were seen. The sandstone rocks are featured with light REE enrichment and heavy REE deficiency. All above indicate that the provenance is originating the southwest paleouplift margin.(3) It is demonstrated from sediments color, lithology, rock texture, sedimentary structure, well logging that in the sedimentary period of Chang8 oil layer group developed a shallow-water delta depositional environment under the background of gentle slope. In the period of Chang8 oil layer group, the mainly subfacies were mostly deltaic plain distributary channel, inter-channel depressions, deltaic front subaqueous distributary channel, river mouth bar, distal and inter distributary bay deposit. Due to the lake level changes frequently, previous distributary channel and river mouth bar are easy to be eroded and scoured by later distributary channel. It will lead to a low development of the distributary channel and mouth bar and a well-developed thick amalgamate channel in delta front and delta plain in study area, which composed the skeletal sand bodies of deltaic deposit. In addition, gentle terrains, shallow water, lake level changes in study area are the main reason that distributary channel and underwater distributary channel developed into net-like distributary channels in the proximal delta front where multi-phase flood density flow deposition controlled by climate changes.(4) On the data analysis of the methods such as thin section, scanning electron microscope, physical property measurement, mercury-injection test, cathode luminescence, clay mineral X-ray diffraction, the main sandstone types in Chang8 oil layer group are mainly debris-feldspar and feldspar-debris sandstone with high plastic rock debris, low compositional maturity and texture maturity. The pore types of ChangS oil layer group are crude residual intergranular pore, intergranular dissolved pore, intragranular dissolve pore, intergranular micropores and microfracture. The pore structure is fine punctate and throat and the physical property is low porosity and penetrability. It is indicated that the evolution of the reservoir mainly resulted from compaction, corrosion, cementation and metasomatism, which belongs to stage A of middle diagenesis phase and part of it belongs to stage B of middle diagenesis phase.(5) Guided by the sedimentation and diagenesis affected on the reservoir development, original sedimentary fabric analysis shows that a higher content of plastic lithic, kaolinite, illite and carbonate in reservoir. Fine particle diameter and poor sorting each has a negative correlation with reservoir property. Among them, kaolinite and carbonate content negatively correlated with reservoir property, which is the main effect on poor reservoir properties. Higher content of feldspar, chlorite and channels has a positive correlation with reservoir property, respectively, which is the key condition for the development of excellent reservoirs. The early compaction can lead to the decrease of porosity, and then cementation. However, later dissolution contributes to the improvement of reservoir property. The pore loss controlled by compression is average for 58% and by cementation is average for 42%. In conclusion, compaction takes the main responsibility for decreasing porosity and permeability of reservoir.(6) Based on the analysis of fluid inclusion, two inclusion types is recognized which are salt-water inclusion and petroleum inclusion. One session with double peak of hydrocarbon filling occurred in the reservoir of Chang8. The double temperature peaks of inclusion are between 90℃-95℃ and 105℃-110℃. Considering the burial history and thermal history, Chang8 hydrocarbon filling developed in Middle stage of Early Cretaceous (±130Ma).(7) On the basis of initial porosity recovery of sandstone reservoir, combined with diagenetic evolutionary sequences, reconstructed the process of porosity evolution, it is assumed that the primary porosity of the Chang8 reservoir rocks was 30.59%, which was reduced by 14.7% by compaction,11.65% by cementation and was increased by 6.36% by later dissolution, resulting the of porosity is 10.6%. According to the porosity evolution curve, at the early diagenetic stage B subage period, after the porosity decreased to 13.7%, the reservoir began to tightness during Late Jurassic period. It is before the hydrocarbon filling. Therefore, it is believed that hydrocarbon filling developed after the compactification of reservoir.(8) Based on the sedimentation and diagenesis on reservoir development, it is considered that reservoir densification of Chang 8 sandstone is fundamental caused by plastic debris sediments with higher content, and finer black mica content within the original sediment. The main reason is for mechanical compaction due to rapid tectonic subsidence after deposition. While late pervasive carbonate cementation is the secondary cause of densification. However, kaolinite precipitation in the pore is unable to be effectively ruled out which limits the improvement of pores.(9) Combined with sequences of diagenesis and porosity evolution, process of Chang 8 reservoir reservoir densification is restored. The early diagenetic stage A is mainly by mechanical compaction, developing leaf shaped chlorite coat on particle surface in form of thin film. The original porosity decreases sharply under strong mechanical compaction. In early diagenetic stage B, authigenic kaolinite and illite smectite mixed layer clay minerals are developed. Secondary enlargement of quartz, and appearance of carbonate cementation result in poor reservoir properties and densified reservoir. During the middle diagenetic phase A, organic matter enters into the mature stage. Hydrocarbon filling brings a large number of organic acid, feldspar, other aluminum silicate and early formation of carbonate cement dissolution in great quantities. A large number of secondary intragranular dissolution pores, intergranular dissolved pore, mold hole, native hole and a small amount of microcracks are formed, improving the reservoir property. With the increase of buried depth, organic matter is highly matured in middle diagenetic phase B. Hydrocarbon filling has been diminished significantly. Diagenetic environment transforms to the weak alkaline environment, quartz secondary enlargement continues. Carbonate cement filling pores are locally appeared. Cementations together with compaction eventually result in present poroperm characteristics.(10) Based on sedimentation, diagenesis on the control of reservoir development, it is hold that those layer with higher feldspar lithic content; coarser clastic particles and dissolution development, together with thick sanbody of distributary channel are high quality reservoirs.(11) Through the analysis of reservoir physical property and cluster analysis of reservoir parameters, reservoir classification evaluation standards of Chang 8 reservoir group has been presented. Three kinds of reservoir can be divided, namely type I (good), type Ⅱ (better), type Ⅲ (poor) with type Ⅱ most developed in the study area. Type I accounts for about 9.46% of overall reservoir, type Ⅱ for 68.92% and type III for less than 21.62%. |
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