Theoretical Calculation Of Secondary Porosity Arising From Dissolutions Of Feldspars In Chang 8₁ Formation Of Maling Oilfield, Ordos Basin

Chang 81 reservoir in Maling Oilfield in Ordos Basin is a new breakthrough in oil-gas exploration. Secondary porosities are important for Chang 81 reservoir. Among them, those secondary porosities originating from dissolutions of feldspar are dominant.This study proposed a new method to calculate the content of secondary porosities originating from dissolutions of feldspar based on thermodynamic principle. The characteristic of Chang 81 reservoir and the formation mechanism of secondary porosities were thoroughly investigated firstly using the examination of casting thin sections, scanning electron microscope, cathodoluminescence (CL), electron microprobe, laser Raman spectroscopy, and microthermometry analysis techniques. Then, the new calculation method was applied to Chang 81 reservoir coupling with data obtained from previous analysis techniques and success was achieved. Main results of this study are:(1) Chang 81 reservoir of Maling oilfield mainly consists of feldspathic sandstone and feldspathic lithic sandstone. Clastic particles mainly consists of feldspar, quartz and lithic. Most of clastic particles in this area are medium-fine grain, medium splitting of deposit, medium psephicity, granular support. The textural maturity of sandstone in Maling area is middle whereas the compositional maturity is low.(2) Residual intergranular pore is the main type of primary porosity and intragranular dissolved pore is the main type of secondary pore of Chang 81 reservoir. The compaction and cementation in process of diagenesis have a damaging effect on the porosity, whereas the dissolution of existed minerals is benifical to the porosity. Dissolutions of feldspar are widespread in Chang 81 reservoir coupling with the depositing of autogenetic illite. With the aid of microthermometry and Laser Raman spectroscopy analysis of saline fluid inclusions existed in clastic quartz, it was proven that secondary porosities in Chang 81 reservoir of Maling are mainly arising from the dissolution of feldspar by organic acids and CO2 from thermal evolution of organic matter.(3) Provided that deeply buried reservoir is close system for dissolutions of minerals and based on data obtained from previous analysis techniques, contents of secondary porosities originating from dissolutions of feldspar in Chang 81 reservoir were calculated. The consistence between calculated values and measured values of secondary porosities confirms the new method proposed by this study. Results of this study are valuable for the exploration and exploiting of petroleum in Maling Oilfield.