**5. Discussion**

The basement reservoirs of carbonate rocks that are discovered in the Jizhong Sub-Basin in the Bohai Bay Basin are mainly dolostones and limestones from the Mesoproterozoic, Neoproterozoic, and Lower Paleozoic periods [38]. Reservoir properties determine the oil and gas accumulation in these reservoirs [17,71,72]. Initial structures and pores of carbonate rocks in these basements are mostly transformed by recrystallization, dolomitization, and tectonism, and has formed the interconnected composite reservoir system consisting of pores and fractures [18,19]. Qiao et al. (2002) studied the relationship between the porosity and burial depth of 32 carbonate basement reservoirs in the Bohai Bay Basin [73]. They found that the porosity does not decrease significantly with the increase of burial depth, while the average porosity generally varies between 5% and 6%, with a negligible change. This infers that because the height variation is not significant in these reservoirs, pores are less affected by the buried depth, and a relatively larger amount of porosity can stay intact to provide a suitable reservoir quality [71]. Moreover, a single basement reservoir has a relatively uniform pressure system and small

changes in fluid properties, and the lithology distribution in the reservoir is relatively stable [9,18,19,38]. However, the above research results show that the development of fractures in these reservoirs has strong heterogeneity. Observations from outcrops, cores, borehole image logs, and thin sections show that these fractures are usually connected (Figures 4 and 6–8). We speculate that the development of opening-mode fractures improves the e ffectiveness of the storage space, thereby influencing the reservoir's e ffective permeability and oil production in these carbonate basement reservoirs.

Through a comprehensive analysis of lithology, opening-mode fractures, and well-testing data, we found that geological characteristics and oil production in the carbonate basement reservoirs vary regionally within the reservoir unit. Considering Well R10, which is drilled in a typical basement inner reservoir with a total of 6 perforated intervals at a depth of 4095–4142 m (13,435.0–13,589.2 ft), significant di fferences in oil production from each perforated interval is reported (Figure 18). Among them, the V and VI perforated intervals displayed higher oil production, 16.62 and 45.58 tons per day (124.30 barrels and 340.95 barrels per day), respectively, making them the main oil production section of Well R10. These intervals are followed by the II and III perforated intervals, with oil production of 4.94 tons and 6.53 tons per day (36.93 barrels and 48.88 barrels per day), respectively. Finally, perforated intervals I and IV are dry layers without any oil production. The lithologic comparison of di fferent perforated intervals indicates that the I and IV perforated intervals without any oil production are mainly limestones, especially the I perforated interval, which is only limestones. In this regard, other perforated intervals with oil production have di fferent degrees of dolostone content where in the V and VI perforated intervals with the largest quantities of oil production, dolostone and limestone are interstratified (Figure 18).

**Figure 18.** Schematic diagram showing the lithology combination, fracture density, perforated interval, and oil production in Well R10. Oil production refers to the daily production of oil in the well-testing stage. Lithology and fracture data were derived from the borehole image logs, and the data pertaining to perforated intervals and oil production were collected from the Huabei Oilfield database. I to VI refer to the name of perforated intervals, which correspond to the names also shown separately in Figure 19. Unit <sup>t</sup>·d−<sup>1</sup> refers to the average number of tons of oil produced per day.

**Figure 19.** Tectonic fractures in the borehole image logs from Well R10. I to VI represent a part of each perforated interval, respectively. Perforated interval numbers are similar to Figure 18. Red lines represent tectonic fractures that appear as sinusoidal curves.

Fracture interpretations from borehole image logs reveal that opening-mode fractures are well developed at a depth of 4095 m to 4142 m (13,435.0 ft to 13,589.2 ft) in the Well R10, and the average fracture linear density can reach 6.78 m<sup>−</sup>1. Besides, the development degree and dip angle of these fractures in different perforated intervals demonstrate a large discrepancy (Figure 19). In this aspect, comparing the oil production with the fracture linear density of each perforated intervals, a positive correlation can be found, which means higher fracture linear density can lead to better oil production (Figures 18 and 19). Considering II, III, V, and VI perforated intervals with higher oil production, opening-mode fractures are developed better, and their dip angles are usually greater than 45◦ (Figure 19II–VI). However, opening-mode fractures are developed less in the I and IV perforated intervals where layers are dry (Figure 19I,IV). In particular, the dip angles of these fractures in IV perforated interval are lower than 45◦, and some are even near horizontal.

Geological characteristics of these perforated intervals reveal that the primary reason for different oil production is the varying degree of regional development of opening-mode fractures. Specifically, oil production grows with the increasing of fractures density (Figure 20). In the intervals where the

dolostone and limestone are interstratified or dolostone makes up the primary lithology, fractures are generally more developed, thus the oil production is higher. However, in the intervals with higher quantities of limestone, the development of opening-mode fractures is relatively poor, and the oil production is lower as well. In addition, tectonic fractures are the dominant type of natural fractures in these basement reservoirs, and their effectiveness varies with the dip angles where tectonic fractures with smaller dip angles exhibit smaller aperture because of the overburden stress. This has caused their connectivity to become relatively poor, thus, their contribution becomes less to oil production [50,74,75]. Also, when the dip angle of tectonic fractures becomes larger, aperture increases and their contribution to oil production enhances. Therefore, it is deduced that tectonic fractures play a major role in the quality of basement reservoirs with carbonate rocks in the Jizhong Sub-basin, and their development has a significant impact on the oil production from these reservoirs.

**Figure 20.** The relationship between the average oil production and the average fractures density in the perforated intervals. The average oil production refers to the daily production of oil in the well-testing stage in every meter perforated interval. The average fracture density is the linear density of opening-mode fractures in the perforated interval from the borehole image logs. Unit <sup>t</sup>·d−1·m<sup>−</sup><sup>1</sup> refers to the average number of tons of oil produced per day in one-meter perforated interval.
