Abstract
The eighth member of Upper Triassic Yanchang Formation (Chang 8) is the major oil reservoir unit in Jiyuan oil field, though with the high potential for oil exploration. The Chang 8 sandstones are characterized with low porosity, low permeability and strong microscopic heterogeneities due to the complex deep-burial diagenetic history. Detailed petrological studies by thin section, X-ray diffraction, scanning electron microscopy, core analysis have been used to investigate the lithogology characteristics, diagenesis, diagenetic minerals and their coupling impacts on reservoir property. The results show that Chang 8 sandstones comprise fine to mediumgrained subarkoses, feldspathic litharenites. The pore systems are dominated by remaining primary intergranular pores, secondary dissolution porosity and micropores. Then, five diagenetic facies were divided in Chang 8 sandstones based on the type and degree of diagenesis, diagenetic minerals assemblages and their coupling effects on the reservoir quality. They consist of grain-coating chlorite weak dissolution facies, unstable component dissolution facies, tight compaction facies, clay minerals filling facies and carbonate cementation facies.The well logging response characteristics of various diagenetic facies are summarized on Gamma Ray (GR), Density Logging (DEN), Acoustic (AC), Compensated Neutron Logging (CNL), and True Formation Resistivity (RT) by translating diagenetic facies to well log responses, the diagenetic facies were defined by a set of log responses, and porosity, permeability ranges for each diagenetic facies were determined from core analyses. Well log data of Luo 13 and Chi 212 are processed to evaluate the accuracy of the predictive model. The diagenetic facies are predicted on the vertical profile based on the generated model. Predicted distribution of diagenetic facies precisely coincide with the microscopic observations, and diagenetic facies in Chang 8 sandstones are generally locally distributed. Tight compaction and carbonate cementation diagenetic facies mainly correspond to the non-reservoir and dry layers of Chang 8 sandstones, layers with higher oil potentials are mainly developed in the grain-coating chlorite weak dissolution and unstable component dissolution diagenetic facies. By translating diagenetic facies to well log responses, diagenetic facies and reservoir properties of intervals that lack core control could be predicted with the same or similar log responses.
Highlights
The North China Craton has been the focus of several recent studies in terms of understanding the geochronologic and tectonic evolution of China and adjacent continental fragments (Ji et al, 2008)
The Upper Triassic Yanchang Formation is the dominant Mesozoic oil oil-source rocks and reservoir rocks in ordos Basin (Ji et al, 2008), and the Yanchang Formation consisting mainly of lacustrine, deltaic and fluvial deposits, is divided into Chang 1 (1st member of Yanchang Formation) to Chang 10 oil layers from bottom to top based on marker beds, lithological association and sedimentary cycles, among these subsections, Chang 7 was deposited in a deep lacustrine environment (Duan, 2012; Duan et al, 2013), Chang 10 to 7 oil layers were deposited in lake transgression period; Chang 6 to 2 oil layers were deposited in lake regression period; Chang 1 oil layers was deposited during peneplain period (Qiu et al, 2014)
Different diagenetic reservoir facies have discrepancies related with macro petrological and mineralogical characteristics. Such discrepancies result in different response characteristic of diagenetic facies in different logging curves and their combinations, which are the theoretical basis of diagenetic facies recognition and detection using well log curves (Zhang et al, 2012; Shi et al, 2011; Lai et al, 2013c), in other words, diagenetic facies can be defined by a set of log responses, with the value ranges of bulk density, neutron, sonic, and Gamma Ray (GR) logs (Ozkan et al, 2011)
Summary
The North China Craton has been the focus of several recent studies in terms of understanding the geochronologic and tectonic evolution of China and adjacent continental fragments (Ji et al, 2008). Though with the high potential for oil exploration, the Yanchang Formation, is a typical sandstone reservoir with low porosity, low permeability and strong microscopic heterogeneity, which is famous for known as a whetstone (Zeng and Li, 2009). Diagenetic facies, which is the combination of types and degree of diagenesis, diagenetic minerals assemblages, determines the formation and distribution of pore type sweet spot in tight sandstones (Zou et al, 2008; Lai et al, 2013a). We intend to build the diagenetic facies and analyze the dominant factors affecting the reservoir property based on the type and degree diagenesis, diagenetic minerals. The model generated can be used to predict the likely diagenetic facies in a single well which lack core control
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