Genetic Analysis Of Deep Marine Tight Sandstone High-quality Reservoirs

The Lower Paleozoic clastic rock with favorable oil and gas geology conditions is the key exploration area in the Middle East.The Devonian Jauf sandstone has a large thickness and good reservoir conditions,but the research on its exploration is restricted to Saudi Arabia and Bahrain.It is found in the uplifted area of Qatar that the Jauf group remained good physical properties under the background of low permeability in the Paleozoic.Hence,carrying out the researches on the reservoir characteristics,sedimentary facies,diagenesis and evolution of the Jauf Formation is of great significance on the causes of the formation of high-quality tight sandstone reservoirs in the area.Based on core,logging data and analysis and test data,sedimentary facies and reservoir characteristics are studied in this paper.It is believed that Jauf sandstone is divided into D3 B and Jauf Res.segments: 1)The overall development of the bank-shore-shallow continental shelf phase can be further subdivided.It is divided into five sedimentary subfacies: Foreshore,Nearshore,Backshore,Offshore Shelf and transition zone.2)The reservoir lithology of Jauf Res.section is mainly quartz sandstone,lithic quartz sandstone and feldspar quartz sandstone,and D3 B section mainly develops microcrystalline dolomite;the pore type mainly develops primary pores and secondary pores with almost no cracks.The ratio of primary pores is 85%,secondary pores are 14%,and microfractures are 1%.Jauf Res.segment far better than D3 B,Jauf Res.porosity of 3.4% ~ 22.3%,permeability of 0.1mD ~ 7.9mD,D3 B porosity of 0% ~ 17.3%,permeability of 0.1mD ~ 2.9 mD,and the proportion of primary pores is much larger than secondary pores.3)The main diagenesis of the Jauf Formation is mechanical compaction,cementation and dissolution;most of the diagenesis destroys the formation of the Jauf Formation;the current diagenetic stage has reached the B-stage of the diagenesis.The causes of high-quality reservoirs are mainly analyzed from the perspective of primary pores and secondary pores.1)The primary pores are mainly developed in the high-energy shoreline environment.Large particle size,well-grounded and well-sorted,quartz sandstone without heterobasic support is more conducive to the formation and preservation of primary pores;the target layer soon enters the rapid burial stage after deposition,and the pore water is too late to drain,resulting in under-compaction and high pressure;an illite film is produced at the edge of quartz grains,which hinders the secondary growth of some quartz and the development of cementation.2)Secondary pores mainly occur in feldspar quartz sandstones.In regions where the feldspar content is high,the better the physical properties,the later hydrocarbon material brings in a wealth of organic acids,which further dissolves the existing unstable space and forms the edges and internal dissolution holes of the rock particles.It is not ruled out that some secondary pores are dissolved by atmospheric fresh water,because the Jauf Formation is located in the lower part of the Carboniferous system where the Hercynian unconformity surface is missing,and feldspar weathering can be observed.