Smaller Extension Of Ortos Basin Gas Field, Extend The Area Of Reservoir Evaluation, Upper Palaeozoic And Favorable Area Prediction

Based on former findings and using theories like modern sedimentology, lithofacies paleogeography and reservoir geology, along with many testing and analyzing methods, this paper studies the upper palaeozoic strata, structure, sediment, and depositing characteristics of the Zichang-Yangchang Area in Yanchang Gasfield, on which it bases the perdictions about profitable areas. The chief dicussions include:a,the gas strata of Lower Permian Dyas in this area are divided into Shihezi Group and Shanxi Group, which are further classified. Shihezi Group consists of four fragments from He5to He8and Shanxi two fragments:Shanl and Shan2, of which He8, Shan1, Shan2and Benxi Group of carboniferou system are the main gas strata;b,the structural characteristics are:the He8, Shanl, Shan2and Benxi Group are slightly tilted to the west, at an angle of one degree; the nose structrue inside is relatively undeveloped;c,the He8, Shanl, Shan2and Benxi Group are located at the area sedimented by underwater distributary river and estuary sandstones in the frontier delta. Sedimentary microfacies are mainly underwater distributary sandstones and distributary interchannel siltstones as well as mudstones; Benxi Group blongs to marine barrier island-tidal sedimention;d,sandstone core samples from main fragements in the the upper palaeozoic strata have showed that the porosity is between0.1%and20.0%and the permeability is between0.01and10.0×10-3μm2. Permeability varies according to the porose development, which form a proportional relationship to each other. Besides, the porose pentad structural difference of gas deposit is decisive to permeability. The samples are all low in porosity and permeability, at smaller than4.98%and1.1×10-3μm2respectively on average. Therefore the area is composed of dense reserviors low in porosity and permeability;e,priliminary evaluation standards of reserviors are established:the Ⅰ and Ⅱ Reserviors are characterized by middle-high bars and bell curves in electrofacies description; the sedimentary types are underwater distributary river and sand flat microfacies, with porosity at5%-18%and overal permeability0.01×10-3μm2-1×10-3μm2. The Ⅲ Reservior is characterized by low detentions, or middle-low finger or bayonet peaks in electrofacies; the sedimentary types are underwater distributary interchannel and mixed flat microfacies. The Ⅳ Reservior is characterized by low bell curves in electrofacies; the sedimentary types are underwater distributary flank or natural levee microfacies. The III and IV Reserviors are smaller than5%at porosity and permeability at0.001X10-3μm2-0.01×10-3μm2;f,the gas reservior features are:the gas dry coeffecienct of He8, Shan1, Shan2and Benxi Group is0.99, with no H2S in its composition. It is non-sulphur gas. The water in these fragments is mildly acid (the pH is5.36), and CaCl2in type (at Sulin Classification). The overall mineralization degree is12.94～210.09g/L and the average mineralization degree is69.86g/L;g,the reservior type is a "quasi-consecutive dense sandstone gas reservior", according to the combined decduction from the layout of reserviors, the distribution and anisotropy in water and gas, the pastry form of reservior distribution as well as the dense properties in the area;h,the paper has concluded that Benxi Group and Shan2, which belong to the I Reservior, are the most profitable; Benxi, Shan2, which belong to the II Reservior, are potential targets; and Shanl, He8, which belong to the I Reservior, are profitable.