**1. Introduction**

Basement reservoirs usually refer to traps that accumulate oil and gas in topographic uplifts of basement rocks under unconformities, which are covered by younger sediments [1–4]. Based on their location in topographic uplifts, these reservoirs are divided into weathering crusts and inner reservoirs [5–7]. Basement rocks include metamorphic rocks and some volcanic and carbonate rocks [4,8–10].

As an important type of unconventional petroleum system for further exploration and development, basement reservoirs are receiving increasing attention worldwide, and hydrocarbons are being exploited from them in areas such as China, North Africa, the USA, and Southeast Asia [1,2,11,12]. Considering recent advancements in production technology and demand for more resources, research to extract oil and gas from the deep inner reservoirs of basements is growing as well [6,13,14]. In the Bohai Bay Basin, basement reservoirs are the primary type of petroleum traps, particularly in the Jizhong Sub-Basin, which is considered as one of the structural units in the Bohai Bay Basin, where 29 out of 43 oil and gas fields are basement reservoirs or related to basement rocks [15]. Volumetric calculations showed proven reserves of 526.83 million tons (3861.66 million bbl) of original oil in place (OOIP) in basement reservoirs, accounting for 51.66% of the total OOIP in the Jizhong Sub-Basin [9]. Carbonate rocks are the primary basement rocks in the Jizhong Sub-basin [16].

Previous studies have shown that primary pores are less in these basement reservoirs, and secondary pores dominate the storage space, mainly composed of dissolution pores and natural fractures [17–19]. Since basement rocks have experienced multiple periods of tectonism and diagenesis, various natural fractures are generally developed, causing significant heterogeneity in such reservoirs [13,20,21]. Like other types of fractured reservoirs, natural fractures are the main flow pathways for hydrocarbons in basement reservoirs [8,12,22,23]. Moreover, natural fractures are a significant component of storage space for petroleum accumulation in these reservoirs as well [24–26], where fractures at di fferent scales can connect pores to control the quality of the reservoir [3,13,27,28].

Despite the vital role that natural fractures play in the reservoir quality as explained above, limited studies are carried out to delineate their significance entirely in basement reservoirs with carbonate rocks. The development of fractures, the main factors a ffecting their existence, and their contribution to the performance are unclear in such reservoirs and demand further investigation.

This study's primary purpose is to characterize natural fractures and understand their role in oil production in the deep inner reservoirs of carbonate basements in the Jizhong Sub-Basin of the Bohai Bay Basin. Therefore, outcrops, cores, borehole image logs, and thin sections were used in a detailed analysis for fracture characterizations, including orientation, dip angle, height, length, density, aperture, and filling. Based on these, lithology, mechanical stratigraphy, and faults were also analyzed to reveal how they would a ffect the growth mechanism of fractures. Moreover, the significance of opening-mode fractures on oil production was presented as well. These e fforts will enable us to understand the heterogeneity of petrophysical properties and reservoir performances in these basements.

## **2. Geological Setting**

#### *2.1. Location and Structure*

The Bohai Bay Basin, which is in the eastern part of China, is a Cenozoic rift basin developed in the basement of the North China Platform [9,29,30] (Figure 1a). The Jizhong Sub-Basin in the northwest is one of the sub-basins in the Bohai Bay Basin. It is distributed in an NNE–SSW direction and covers an area of 32,000 km<sup>2</sup> (12,355 mile2) [31,32] (Figure 1b). A number of NNE–SSW-oriented depressions, uplifts, and slopes have been developed within the sub-basin. Additionally, two transfer zones—Xushui-Anxin-Wenan and Wuji-Shenxian-Hengshui—are developed in the Jizhong Sub-Basin, near E–W and NW–SE strikes, respectively (Figure 1b). The two transfer zones have divided the sub-basin into three regions: northeast, center, and southwest. The extensional faults formed by multistage tectonic movements are developed in this sub-basin, while grabens and half-grabens, from these faults, have created various topographic uplift structures in this sub-basin [33,34] (Figure 2).

**Figure 1.** (**a**) Location of the Jizhong Sub-Basin in the Bohai Bay Basin in China (modified from Zhao et al., 2015) [9]. (**b**) Map showing the faults, structural units, transfer zones, wells, and outcrops in the Jizhong Sub-Basin (modified from Chang et al., 2016) [35]. The fault data was modified from the Huabei Oilfield database. A-A' is the Xushui-Anxin-Wenan transfer zone, and B-B' is the Wuji-Shenxian-Hengshui transfer zone. C-C' represents the position of the cross-section in Figure 2.

**Figure 2.** A geological cross-section in the Jizhong Sub-Basin, showing the structural pattern of the strata (modified according to He et al., 2017) [36]. The position of the cross-section is shown by C-C' in Figure 1.
