Study On Gravity Flow And Tight Reservoir In Sha Ⅲ Member, Southern West Sag Of Liaohe Depression

With the rapid development of global economy and the sharp increase in energy demand, it is not sufficient only depending on conventional petroleum. There is a huge potential and good prospect for unconventional petroleum resource, especially for the compacted sandstone petroleum which is of the most real economic significance currently. At the same time, the development of theories on the gravity flow has experienced ups and downs over more than 60 years since 1950 s when the concept of turbid flow first turned out, in which the genesis interpretation evolved from graded bedding to classical Bouma Sequence and to the doubt about Bouma Sequence as well as the sublacustrine fan deposit mode. It’s of more practical meaning that the sedimentation study has been focused on the sedimentary process and thus the proposed theory of sandy clastic flow has been utilized since the 21 st century, which has simultaneously pushed the gravity flow study to a further stage. This paper gives systematic presentation and discussion on the gravity flow sedimentation and compacted sandstone reservoir of Sha-3 Member in the south section of Western Sag in Liaohe Depression.The gravity flow deposits occurs in Sha-3 Member in the south section of Western Sag in Liaohe Depression with the following advantages in geology settings: high mountains and deep valley with steep slope, sufficient source materials, warm humid climate, deep water in the lake basin, active faulting and various inducement of earthquake and volcanism.Gravity flow sediments are generally coarse and the lithofacies can be classified into 4 categories with 14 subcategories. Gravels, sands and muds are mixed in the rocks; boulder clay and mud debris are widespread with different colors and shapes; a number of sedimentary structures such as massive bedding, graded bedding and Bouma Sequence reveals the genesis of the gravity flow; the varied phenomena of slump deformation structure, liquefied sandstone vein, water escape structure, micro-faults, ball-pillow structure, load cast and flame structure reveal the impact of earthquake.Mud boulder(debris) is incorporated in the gravity flow sedimentation through the whole process, which can be used as the tracer of the gravity flow sedimentation and may exist in any kind of rocks such as conglomerate, sandy conglomerate, sandstone and mudstone, etc. The mud boulder(debris) with oxidation tint is sourced from the land, with good(ellipsoidal) sphericity and regular form; while the mud boulder(debris) with a reduced color is sourced from the lake, lacerate and messy; different macroelements, microelements and rare earth elements occur in these two types of mud boulder(debris), between which is the mud boulder(debris) with a transitional color.Three types can be divided for the gravity flow sedimentation in terms of the ways it takes place, i.e., sliding and slumping flow, debris flow, turbid flow. Debris flow can be subdivided on the basis of the grain size and supporting mechanism into pebbled debris flow, sandy debris flow and argillaceous debris flow. The sedimentation facies in the research area is mainly midfan subfacies in the sublacustrine fan including braided channel microfacies and inter-channel and channel front microfacies, as well as outer fan subfacies and semideep-deep lacustrine facies. Multi factors controlled gravity flow mode is therefore developed according to the identification of the sectional facies, plane facies and evolution rules.Sha-3 Member in Shuangtaizi area in the south section of Western Sag belongs to the deep reservoir(>3500m) which is generally tight with low porosity(averaging 11.65%) and low permeability(averaging 10.9×10﹣3μm2). It’s dominated by the occurrence of feldspathic litharenite and lithic arkose, poorly sorted and rounded, with main interstitial materials of clay minerals and carbonate cements. Intergranular pores, intragranular pores, micropores, fissures and their combination can be observed, among which secondary erosion pores are main types followed by intergranular primary pores and micropores secondly. Throat radius is generally below 1?m and the common throat combination is mainly medium pores with medium throat, medium pores with fine throat, fine pores with fine throat and fine pores with micro throat.Because of the large buried depth, the reservoir of Sha-3 Member in Shuangtaizi area is subject to strong diagenesis, which has reached middle diagenetic Stage A2- middle diagenetic stage B. The particles are mainly in line contact and point-line contact, with major carbonate cementation and then secondary outgrowth cementation of clay minerals and quartz as well as feldspar. The carbonate cementation is mainly composed of(ferroan) dolomite and(ferroan) calcite. Quartz and feldspar outgrowth scale generally reaches Class Ⅱ-Ⅲ. Kaolinite as a clay mineral has generally been transformed into illite and chlorite. The content ratio of illite smectite mixed clay(I/S) is below 15%. The secondary pores may mainly result from the dissolution of feldspar cause by organic acid and some dissolution of rock debris and carbonate can be observed at the same time. Metasomatism is mainly represented by later(ferroan) carbonate replacement of quartz, feldspar and debris. The reservoir tightness is due largely to the poor pore-throat structure caused by strong compaction and strong cementation and to relatively weak dissolution in the later stage.The development of the reservoir is controlled by sedimentation conditions and diagenesis. The braided channel subordinated to the midfan subfacies in the sublacustrine fan is a favorable place for reservoir development, thanks to the thick sand bodies and small shale content as well as coarse grains. Quantitative calculation on the diagenesis indicates that the percentage of loss suffered during the compaction and cementation for compact reservoir is more than 30%, while the porosity gain due to the dissolution is below 3%. Four secondary pore zones can be observed vertically through the reservoir section. The former three of them are associated with organic acid dissolution caused by the thermal evolution of varied organic material, and the last just may be related to clay minerals alteration, CO2 partial pressure rise, heat cycle convection and deep thermal fluids.The lower limit of porosity and permeability for reservoir in Sha-3 Member in Shuangtaizi area is respectively 5% and 0.9×10﹣3μm2. The reservoir can be divided into four types according to the porosity demarcation of 5%, 10% and 15%. Seen from the plan view, the reservoir turns poorer in quality from the braided channel to inter-channel, channel front and outer fan, in which Reservoir I is generally transited to Reservoir II and Reservoir III. While in section view, the reservoir just turns better in quality upwards. Lower and medium Sha-3 Member in Shuangtaizi area can be classified as Reservoir III, while the upper belongs to Reservoir II and III.The reservoir of Sha-3 Member in Shuangtaizi area generally turns tight during the diagenetic stage A1, in which the deep part began from the sedimentation of the Dongying Formation and the shallow just is subject to densification currently. According to the identification of petroleum accumulation phase and time, the sequence of the densification and accumulation for the reservoir is varied including accumulation-after-densification mode, densification-after-accumulation mode and simultaneous accumulation-accumulation mode.