diagenesis and its effect on reservoir quality of silurian sandstones， tabei area， tarim basin， china

1、Petroleum Science 2007 Vol.4 No.3Diagenesis and Its Effect on Reservoir Quality of Silurian Sandstones, Tabei Area, Tarim Basin, China Zhang Jinliang, Jia Ying and Du Guilin (School of Geosciences, Ocean University of

2、 China, Qingdao, Shandong 266003, China) Abstract: The diagenetic processes of the Tabei sandstones in the Tarim Basin include compaction, cementation (quartz overgrowths, calcite, clay minerals and a minor amount of pyr

3、ite), and dissolution of the feldspar and calcite cement. Porosity was reduced by compaction from an assumed original 40% to about 22.1%. Cementation reduced porosity to 26.6%. The Tabei sandstones lost a little more por

4、osity by compaction than by cementation. Quartz cementation, especially syntaxial quartz overgrowth, is a major cause of porosity-loss in many reservoirs in moderately to deeply buried sandstone. Calcite cementation play

5、ed a key role in the porosity evolution of sandstones. At the early stage of burial, the early calcite cement occupied most of the pore spaces resulting in significant porosity. On the other hand, some primary porosity h

6、as been preserved due to incomplete filling or the presence of scattered patches of calcite cement. In addition to calcite, several clay minerals, including illite and chlorite occurred as pore-filling and pore-lining ce

7、ments. The pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of syntaxial quartz overgrowths. Illite, which largely occurred as hair-like rims around the grains and bridges on

8、 the pore throats, caused a substantial deterioration of penetrability of the reservoir. Calcite cement dissolution was extensive and contributed significantly to the development of secondary porosity. Key words: Tarim B

9、asin, diagenesis, secondary pore, reservoir quality 1. Introduction The Tarim basin, surrounded by Tianshan Mountains and Kunlun Mountains, is located in the southern part of Xinjiang Uygur autonomous Region and is

10、 the largest sedimentary basin in China, covering 56×104km2. As an important basin, Tarim Basin experienced several periods of tectonic movement and depositional cycles, with source rock sections developed in dif

11、ferent strata. During the Silurian period, the stable central basin was the Tarim plate, the basin was an active continental margin and the southern basin was the Aerjin marginal uplift and the Kunlun foreland thrust

12、 faulted zone. The Tabei area lies in the north of the Tarim basin. The Qunke rift zone and Akesu lie in its east and west, respectively. To the north, it is Erbatai rift and to the south, it is transitional to the T

13、azhong area. Fig. 1 Stratigraphic succession of Silurian in the Tabei Area Diagenesis and Its Effect on Reservoir Quality of Silurian Sandstones, Tabei Area, Tarim Basin, China Vol.4 No.3 3Fig. 3 Types of sandstone fr

14、om the Silurian reservoir, Tarim Basin in middle Sk (left) and upper Sk (right) (After Folk, 1968) formations: the middle Sk, the lower and upper St sandstones are well sorted, but the upper Sk sandstones are medium to

15、 well sorted. 3. Diagenesis of sandstone and its effect on reservoir quality 3.1 Compaction Compaction and cementation of sediments played a major role in reducing porosity (Fisher, et al., 1999). Conspicuous evidenc

16、e of mechanical compaction includes the fracturing of quartz and feldspar grains (commonly healed by authigenic quartz) and by the deformation of ductile grains. Chemical compaction occurred by pressure dissolutio

17、n both along intergranular contacts and stylolites. In this study, the closely packed sandstones shows three types of grain contacts in Tabei reservoirs, which include point-, long (line contact)- and concavo-conve

18、x contacts. Compaction is limited in sandstones cemented by abundant eogenetic silica or carbonate cementation. Locally, in sandstones cemented by microquartz rims, pressure dissolution is limited to intergranular su

19、tured and concave-convex contacts with rare, low-amplitude (0.1 mm). The sandstones show the effects of both mechanical and chemical compaction by the loss of initial porosity, ductile and brittle deformation of grai

20、ns. Mechanical compaction is the dominant process, whereas chemical compaction is scarcely found, resulting in a major loss of primary porosity. When assessing the diagenetic modification of primary porosity, it is

21、useful to separate the effects of compaction from the effects of cementation. The relative importance of the two processes can be visualized by the use of Ehrenberg’ s (1989) diagram (Fig. 4). Compaction is limited i

22、n sandstone cemented by abundant silica or carbonate cementation. Cementation increased the framework strength of the sediments and preserved primary porosity from compaction reduction. All the time of deposition, cl

23、ean well sorted sandstones have an initial porosity value. This value was assumed for the studied sandstones. After deposition, that porosity was reduced by compaction and cementation. The total amount of porosity lo

24、st by compaction (COPL) of sandstones can be expressed by the following equation (Ehrenberg, 1989). (100 ) ( )100IGV OP IGV COPL OP IGV× ? × = ? ?Where OP= the original porosity (40%) and IGV=the intergranul

25、ar volume (intergranular porosity + pore filling cement). The amount of porosity lost by cementation (CEPL) of sandstones can be expressed by the following equation: ( ) CEM CEPL OP COPL IGV = ? ×The results of e

26、quations show that more porosity was lost by compaction than by cementation. The average is 17.9% (up to 28%) porosity loss by compaction, average 13.4% (up to 29) porosity loss by cementation. Fig. 4 Diagram showing

27、 the relative importance of compaction versus cementation to porosity development in the sandstones (After Ehrenberg, 1989) 3.2 Cementation Fig. 5 Types of authigenic mineral illustrated by histograms in Silurian re