Influences Of Authigenic Chlorites On Sandstone Reservoir Quality In Chang-8 Member Of The Upper Triassic Yanchang Formation In Jiyuan-Huaqing Area, Ordos Basin

Based on the major and special project of national science and technology in the Eleventh Five Year Plan -"Reservoir characteristics of mesozoic Yanchang Formation in Ordos Basin", the occurrence, distribution, formation mechanism of authigenic chlorites and their influences on reservoir properties and pore structure in chang-8 Member of the upper Triassic Yanchang Formation in Jiyuan-Huaqing area, Ordos Basin, were studied by thin section analysis, XRD, SEM, physical property analysis and mercury intrusion analysis and some conclusions are as follows:(1) Authigenic chlorites mainly present in the form of pore-lining cement is with the highest content in authigenic clay minerals, often coexisting with authigenic kaolinite filled in inter-granular pores.(2)According to characteristics of occurrence and distribution, the diagenetic mechanisms of the formation of authigenic chlorites are summarized:(a)Abundant neutral–basic volcanic materials, including neutral–basic lava and pyroclastic materials in sandstones before buried provided important material sources for the development of authigenic chlorites;(b)Unloading of iron was related to meandering river delta and fan delta in Jiyuan-Huaqing area and certain salinity of lake in Ordos Basin during the deposition of chang-8 Member;(c) Authigenic chlorite was formed roughly in early stage of early diagenesis or even earlier. Precipitation of chlorite rim began before the dissolution of the majority of feldspar (also including other skeleton grains), and the oriented growth of these chlorites during the later diagenetic process lasted until diagenetic stage after the precipitation of authigenic quartz;(d) Authigenic chlorites are associated with kaolinite. However, the formation of the two is not at the same time, but with time difference. Authigenic chlorites formed in the diagenetic time with paleotemperature lower 60℃- 70℃. After a large number of organic acids were discharged, the growth of chlorite ceased or slowed down, and kaolinite was precipitated as the main authigenic mineral.(3)Based on the formation time and its relationships with the evolution of pore fluids and the precipitation of kaolinite, pore-lining authigenic chlorites play an important role in the preservation of porosity in sandstone reservoirs by the following mechanisms:(a) Pore-lining chlorites and their overgrowth during buried diagenesis enhanced the mechanical strength of rocks and the ability of resistant to compaction, so inter-granular pores (mainly primary pores) would not be further reduced due to compaction.(b) Pore-lining chlorites isolate detrital surfaces from contact with pore-water, thus preventing nucleation of authigenic quartz. Due to that dissolution of feldspars occurred in a relatively late diagenetic stage when organic acids were formed by thermal evolution of organic matters, and precipitation of related authigenic quartz occurred later, with limited ability of resistant to compaction, inhibition of nucleation of quartz by the pore-lining chlorite was positive.(4) 102 samples with varied authigenic chlorite content but without kaolinite were selected to study relationships between authigenic chlorite content and porosity -permeability curve. Corresponding porosities for obtaining the permeability of 0.01, 0.1, 1 and 10×10^-3μm² are gained, the following conclusions are reached by comparison of the results:(a)Overall, with the increasing of authigenic chlorite content in sandstones, porosity tends to increase to obtain the same permeability. However, for different ranges of permeability and contents of authigenic chlorites, porosities increase in different degrees to get the same permeability.(b)Because of permeability of most samples (about 70 percent) are between 0.04×10^-3μm² and 0.8×10^-3μm², thus porosities for obtaining the permeability of 0.01×10^-3μm² and 0.1×10^-3μm² are with relatively high credibility.(c) Based on the above principle, when considering the limit of porosity for defining the permeable sandstone or calculating reserves, the content of authigenic chlorite should be considered.(5) The relationship between authigenic chlorite content and the parameters of pore structure was studied on 10 samples. The results are as follows:(a)When sandstone porosity changes little, with increasing authigenic chlorite content, the permeability of the sandstone decreases while drainage pressure and median pressure increase, and the corresponding throat radius decreases obviously; meanwhile, throat mean value and the number of throats in different types also decrease significantly.(b)Although the increasing of authigenic chlorite content would ameliorate the sorting and heterogeneity of throats, the number of large throats would reduce correspondingly.(c) The increasing of authigenic chlorite content would remarkably reduce ejection efficiency, which may indicate that oil recovery may be lower when the content of pore-lining authigenic chlorite is higher in rocks.(d) Increasing pore-lining authigenic chlorites would reduce the quality of sandstone reservoirs and affect hydrocarbon exploitation under the prerequisite of similar porosity.