The diagenetic environment is vital in controlling reservoir quality by influencing diagenetic processes, especially porosity evolution. The Zhuhai Formation is a critical exploration target in the Huizhou Sag. However, there are considerable differences between the reservoirs in the west (Huixi) and east (Huidong) Huizhou Sag with only a 400 m difference in burial depth. To investigate causes of these differences, scanning electron microscopy (SEM), cathodoluminescence analysis, fluorescence analysis, and fluid inclusion analysis are employed for the petrological and diagenetic characterization of the target formation. Moreover, image analysis software is used to qualitatively evaluate the impacts of diagenesis on porosity. The results show that the digenetic environments in Huixi and Huidong are different. Huixi is characterized by a high content of feldspar with a stronger dissolution phenomenon than that observed in Huidong. The main cement types in Huixi are siliceous cement and kaolinite. Moreover, Huixi had a low palaeo-temperature, low palaeo-salinity, and low palaeo-pH diagenetic environment. In contrast, Huidong is accessible via deep faults and rich in lithic fragments which show visible deformation. Moreover, cements in this area include carbonate, gypsum, quartz overgrowth, kaolinite, and chlorite. This evidence confirms that the diagenetic environment in Huidong was a high palaeo-temperature and high palaeo-salinity environment with high palaeo-pH. Additionally, the palaeo-pH evolved following the sequence of weakly alkaline, acidic, alkaline and acidic. Microscopy evidence indicates that the Dongsha Uplift was a likely source of Huixi sediments during the deposition of the Zhuhai Formation. The determination of the porosity evolution shows that compaction and lithic fragment content are the main controlling factors on the reservoir quality of the Zhuhai Formation.

Based on seismic reflection characteristics, lithologic associations, and basin structure evolution stages, the Banuwati Formation, Zelda and Gita Members in the Oligocene could be divided into six third-order sequences. The new sequence framework has been established in the Sunda and Asri Basins. Sequence relationship between wells and seismic data has been implemented in the basin scale through the use of synthetical seismic records of 14 wells. Depositional systems, such as delta, fan-delta, nearshore subaqueous fan, and fluvial, are identified in the Banuwati Formation, Zelda and Gita Members. Alluvial fan, subaqueous fan, and fan delta occurred on the steep slopes of the basins. Deltas were mainly developed on the gentle slopes of the basins. There were three separated small basins with fluvial and shore-shallow lake deposition in the Banuwati Formation (SQ1). During the Lower Zelda (SQ2) sedimentation, the former separated small basins enlarged in a certain extent where delta and subaqueous fan were developed. Major lacustrine facies under deepwater environments and deltaic facies developed in the Upper-Lower Zelda and Middle-Upper Zelda (SQ3, SQ4). The sedimentary situation was almost the same as the Middle Zelda, but the water depth became shallower during Upper Zelda sedimentation. Later, fluvial and swamp developed in the upper part of the Upper Zelda and Gita Member (SQ5, SQ6). In general, basin sedimentary evolution is consistent with tectonic evolution. Finally, the favorable sandstone distribution in the Banuwati and Mid-Upper Zelda Member for further exploration was predicted according to the depositional systems, slope break, and present hydrocarbon occurrence. Three fields or eight favorable plays should be paid attention to for further exploration in the Sunda and Asri Basins, Indonesia.

Xianzhang YANG, Dakang ZHONG Lu ZHOU, Xiaojuan HUANG, Zhenping XU, Yani XIE, Guowei ZHANG, Xueqi ZHANG, Haoyu LUO & Haitao Sun - THE ORIGIN AND CONTROLLING FACTORS OF SWEET SPOTS IN JURASSIC TIGHT SANDSTONES OF KUQA FORELAND DEPRESSION, TARIM BASIN, NW CHINA, Carpathian Journal of Earth and Environmental Sciences, August 2021, Vol. 16, No. 2, p. 373 – 381; DOI:10.26471/cjees/2021/016/182

Sandstone porosity is an important factor for hydrocarbon reservoirs. Based on the analysis of statistical porosity data from ten Chinese basins, three different trendlines has been found for basins of different categories. High average porosity reservoir is often developed in the Eastern Rift Basins, occasionally in the Western Superimposed Basins. The sandstone reservoir of Middle Craton Basins is always developed with low average porosity. However, the reservoir porosity decreased with increasing depth vertically with one or two anomalously high porosity belt only in the Eastern Rift Basins. According to these results, we should take basin category as a key factor to make porosity prediction in the hydrocarbon exploration.