Single-well Sedimentary Facies Division of Typical Wells
The classfications of sedimentary facies for the following 8 single wells has been analyzed: Liang3 well, Jianxi1 well, Jian3 well, Jian5 well, Jian6 well, You6 well, Feng2 well and Nan10 well. The single well sedimentary microfacies of Nan10 well, Jian3 well in the period of E1+2, E31, E32 as well as that of You6 well and Feng2 well in the period of E31, E32 will be discussed in more detail.
Nan10 well in the depression region of western Qaidam Basin is to the south of Nanyishan tectonic belt (Fig. 2). 3158m-4255m is for E32 Formation, 4255m-4566m is for period of E31, and 4566m is the top of the E1+2 Formation, bottom not reached. The lithology of this region, which consists of mainly grey and dark grey calcic mudstone, including some mudstone colored brownish grey, some sandy shale and calcisiltie, was shallow lacus facies deposit. In period of E31 (4566m-4255m), Nan10 well mainly developed braided-river delta plain subfacies which can be divided into distributary channel microfacies and interdistributary bay microfacies(Liu, et al., 2009; Luo, et al., 2009; Zhang, et al., 2006), only sedimentated braided-river delta front subfacies on the bottom. It developed a large lacustrine sedimentation in period E32 around Nan10 well, so Nan10 well developed the subfacies of braided-river delta front, braided-river delta plain and shallow lacus facies from bottom to top.
Jian3 well is located on the top of Jianbei buried structure. The section of 1085m-2376m is for E32 Formation, 2076m-3014m is for E31 Formation, and E1+2 Formation is under 3014m, bottom not reached. It was subfacies of meandering riverbed that sedimented in the period of E1+2. The E1+2 period sedimentated the subfacies of meandering riverbed in the basin, where the lithology are primarily purple and dark purple mudstone, mingled with grey sandy limestone, calcareous sandstone and grayish green mudstone. According to lithology, it can be divided into fluvial channel microfacies, lake flood plain microfacies and fluviatile flood plain sedimentation microfacies. Jian3 well mainly developed braided-river delta plain subfacies during period of E31. The lithology were mainly siltstone, fine sandstone, pebbled sandstone and grey conglomerate, followed by purple mudstone and sandy shale, and a small amount of grey green mudstone. The bottom of Jian3 well is a set of alluvial fan gravel layer which is about 8 meters thick (Zhang, 2007; Wang, et al., 2007). E31 Formation developed four positive cycles of sedimentation from bottom to top and all of them were composed by distributary channel sedimentation and interchannel or interdistributary bay sedimentation. The lithology of E32 (1085m-2376m) section in this well are mainly mudstone and lime mudstone colored grey, grey green, brownish red and mulberry, mixed with superior dark grey limestone, muddy limestone and grey siltstone and fine sandstone. The well developed braided-river delta front subfacies (2376m-2220m) and shallow lacus subfacies （2220m-1085m）. The former one includes interdistributary bay underwater mingled together with sand sheet subfacies and mouth bar subfacies. The sedimentary facies of Jian3 well changed significantly from period of E1+2 to E32. There sedimentated mainly alluvial fan in the period of E1+2, which turned to braided-river delta plain sedimentation when reached E31 Formation. It developed braided-river delta front and shallow lake deposit during period of E32.
You6 well is situated in the south wing of Youquanzi structure in Qaidam Basin. The drilled well is 4616m in depth. E32 Formation covers 4263.5m-2877m and period of E31 Formation is beneath 4263.5m. Period of E31 was mainly braided-river delta plain facies which can be further divided into predelta subfacies, braided-river delta front subfacies and braided-river delta plain subfacies. The lithologic character of E32 Formation is light grey siltstone,calcareous siltstone and limy siltstone, which presented non-isopach with dark grey and grey mudstone. The siltstone is generally 1.5m-3m thick. The calcareous sandstone is 1.5m-2m thick and the thickest is 3m. The sedimentary facies of period of E32 were shallow lacus facies and braided-river delta front facies.
Feng2 well is located in Mangya depression of Qaidam Basin, the axial of Dafengshan fold tectonic. The depth of E32 Formation is 3000m-4410m, E31 Formation is below 4410m. E31 Formation of Feng2 well mainly distributed with alluvial fan deposits, followed by fluvial channel subfacies and flood plain subfacies. The lithologic character of Feng2 well during E32 was mainly alternate strata about crimson, fuscous and grey mudstone, brown and oyster grey siltstone, and calcareous sandstone. Gamma-ray curve was seriously serrated denticle. According to these characteristics, it was defined as flood plain sedimentation. The sedimentary facies of Feng2 well developed from fluvial channel deposit subfacies of alluvial fan deposit to flood plain subfacies between period of E31 and E32.