SUMMARY

Granulometric analysis of the sandstones by thin section - application in studying
the Late Miocene sedimentary environment in the Southeast area of Hà Nội depression

Trần Nghi, Nguyễn Thị Phương Thảo, Đinh Xuân Thành, Lương Hồng Hược,

Nguyễn Thị Huyền Trang, Lê Nam, Đào Trung Hoàn

For reservoir sandstone, the determination of particle size parameters such as average diameter of fractions (Md), sorting coefficient (So) and skewness coefficient (Sk) is very important not only in the lithofacies analysis and sedimentary environment but also in evaluating the quality of reservoir rock. For unconsolidated sediments the granulometric analysis is primarily used a sieve sets and pipette. The article presents the method of measuring and correcting from the percentage content of the measured fractions into true ones. From that condition, all types of sandstone are classified into 3 groups: (1) group 1: the coarsest particle size is 2-1 mm, including 5 fractions (2-1 mm; 1-0,5 mm; 0,5-0,25 mm; 0,25-0,063 mm and <0,063); (2) group 2: the coarsest particle size is 1-0,5 mm, including 4 grain fractions; (3) group 3: has the coarsest particle size of 0,5-0,25 mm, including 3 particles. As the results its were divided into 5 lithofacies groups according to 3 systems tract in relation to 3 phases of sea level change in the southeastern region of the Hanoi depression based on the parameters Md, So and Sk analysing from 100 thin sections, as follows: (1) Lowstand river channel sand facies group (SarLSTN₁³); (2) The lowstand river mouth fan-cone sand facies group (SamrLSTN₁³); (3) Transgressive tidal flat sand facies group (amtTSTN₁³); (4) The highstand tidal flat sandy facies group (SamhHSTN₁³); (5) Highstand river mouth lagoon sandy facies group (SamhHSTN₁³).

Keywords: Refund coefficient, correct formula, cumulative curve, sorting coefficient