Chengyan Lin

Petrographic characterization to build an accurate rock model using micro-CT: Case study on low-permeable to tight turbidite sandstone from Eocene Shahejie Formation

Pore scale flow simulations heavily depend on petrographic characterizing and modeling of reservoir rocks. Mineral phase segmentation and pore network modeling are crucial stages in micro-CT based rock modeling. The success of the pore network model (PNM) to predict petrophysical properties relies on image segmentation, image resolution and most importantly nature of rock (homogenous, complex or microporous). The pore network modeling has experienced extensive research and development during last decade, however the application of these models to a variety of naturally heterogenous reservoir rock is still a challenge. In this paper, four samples from a low permeable to tight sandstone reservoir were used to characterize their petrographic and petrophysical properties using high-resolution micro-CT imaging. The phase segmentation analysis from micro-CT images shows that 5-6% microporous regions are present in kaolinite rich sandstone (E3 and E4), while 1.7-1.8% are present in illite rich sandstone (E1 and E2). The pore system percolates without micropores in E1 and E2 while it does not percolate without micropores in E3 and E4. In E1 and E2, total MICP porosity is equal to the volume percent of macrospores determined from micro-CT images, which indicate that the macropores are well connected and microspores do not play any role in non-wetting fluid (mercury) displacement process. Whereas in E3 and E4 sandstones, the volume percent of micropores is far less (almost 50%) than the total MICP porosity which means that almost half of the pore space was not detected by the micro-CT scan. PNM behaved well in E1 and E2 where better agreement exists in PNM and MICP measurements. While E3 and E4 exhibit multiscale pore space which cannot be addressed with single scale PNM method, a multiscale approach is needed to characterize such complex rocks. This study provides helpful insights towards the application of existing micro-CT based petrographic characterization methodology to naturally complex petroleum reservoir rocks.

Formation of chlorite rims and the impact of pore-lining chlorite on reservoir quality: a case study from Shiqianfeng sandstones in upper Permian of Dongpu Depression, Bohai Bay Basin, eastern China

ABSTRACT The source material, precursor and formation processes of chlorite rims, and impact of pore-lining chlorite on reservoir quality of the Shiqianfeng sandstones, Dongpu Depression, Bohai Bay Basin, China, are studied using an integrated approach, including core observation, point-count analysis of thin-sections, scanning electron microscopy, electron microprobe analysis and cathode luminescence. The petrographic analysis shows that chlorite rims consist of grain-coating chlorite, poorly crystalline pore-lining chlorite and euhedral-crystallised pore-lining chlorite. The chemical composition shows that pore-lining chlorite is mainly Fe-rich with an average of 0.785 for Fe/(Fe+Mg) ratio. Petrographic analysis shows a large amount of volcanic dust (3.0∼16.0 vol%, average of 7.93 vol%) in Shiqianfeng sandstones, which determines the formation of poorly crystalline pore-lining chlorite. Transformation of volcanic dust to smectite rims started with shallow burial depth at an early diagenetic stage, followed by in situ alteration of the smectite rims to poorly crystalline chlorite rims. Euhedral-crystallised chlorite mainly develops in sandstones with high porosity, high permeability and open flow systems. Pore-lining chlorite can inhibit quartz overgrowth but cannot effectively prevent pore-filling of authigenic quartz, carbonate and kaolinite cements, and therefore cannot prevent porosity destruction. However, the occurrence of pore-lining chlorite is a barometer of good reservoir quality and intense hydrodynamic conditions.

Architecture and reservoir quality of low-permeable Eocene lacustrine turbidite sandstone from the Dongying Depression, East China

Abstract The architecture and quality of lacustrine turbidites that act as petroleum reservoirs are less well documented. Reservoir architecture and multiscale heterogeneity in turbidites represent serious challenges to production performance. Additionally, establishing a hierarchy profile to delineate heterogeneity is a challenging task in lacustrine turbidite deposits. Here, we report on the turbidites in the middle third member of the Eocene Shahejie Formation (Es3), which was deposited during extensive Middle to Late Eocene rifting in the Dongying Depression. Seismic records, wireline log responses, and core observations were integrated to describe the reservoir heterogeneity by delineating the architectural elements, sequence stratigraphic framework and lithofacies assemblage. A petrographic approach was adopted to constrain microscopic heterogeneity using an optical microscope, routine core analyses and X-ray diffraction (XRD) analyses. The Es3m member is interpreted as a sequence set composed of four composite sequences: CS1, CS2, CS3 and CS4. A total of forty-five sequences were identified within these four composite sequences. Sand bodies were mainly deposited as channels, levees, overbank splays, lobes and lobe fringes. The combination of fining-upward and coarsening-upward lithofacies patterns in the architectural elements produces highly complex composite flow units. Microscopic heterogeneity is produced by diagenetic alteration processes (i.e., feldspar dissolution, authigenic clay formation and quartz cementation). The widespread kaolinization of feldspar and mobilization of materials enhanced the quality of the reservoir by producing secondary enlarged pores. In contrast, the formation of pore-filling authigenic illite and illite/smectite clays reduced its permeability. Recovery rates are higher in the axial areas and smaller in the marginal areas of architectural elements. This study represents a significant insight into the reservoir architecture and heterogeneity of lacustrine turbidites, and the understanding of compartmentalization and distribution of high-quality sand reservoirs can be applied to improve primary and secondary production in these fields.

Paleogeomorphology restoration and the controlling effects of paleogeomorphology on karst reservoirs: a case study of an ordovician-aged section in Tahe oilfield, Tarim Basin, China

The Tarim Basin is the largest petroliferous basin in northwestern China and contains the Tahe oilfield, the highest hydrocarbon enrichment in the entire basin. Paleogeomorphology has important controlling effects on carbonate reservoir development. Using core data, thin sections and drilling, logging and seismic data, paleokarst was identified and analyzed. The paleogeomorphology of the top low-middle Ordovician was recovered following a comprehensive utilization of the impression and residual thickness methods in combination with unconformity identification. Based on the filling thickness and residual thickness, the karst paleogeomorphic units were classified, and their controlling effects on reservoirs analyzed. The second-order paleogeomorphic units primarily include four types: karst highland, karst gentle⁃slope, karst steep⁃slope and karst basin. The karst gentle⁃slope and karst steep⁃slope are further classified quantitatively into third-order paleogeomorphic units (monadnock, tableland, karst terrace and trench). The results show that the development of the karst reservoir differed in different geomorphic units. In the karst gentle slope, the favorable areas for karst reservoir development are the karst terrace, tableland, monadnock, and the ends and flanks of trench. In the karst steep⁃slope, the karst terrace and tableland are favorable areas. In the vertical vadose and phreatic zones of the gentle-slope region, a large number of caves and underground river systems developed, but in the steep-slope region, only a small number of shafts and isolated caves are observed. This study plays an important role in oil and gas exploration and development.