Karst reservoirs are one type of important marine carbonate reservoirs in China. With the discovery of many large oil and gas fields, such as Jingbian, Tahe, Halahatang, and Shunbei, karst reservoirs are drawing increasing attention from petroleum geologists^{[1,2,3,4,5,6,7,8,9,10,11,12]}. The marine strata of the Sichuan Basin experienced several periods of structural uplifting, and consequently, many karst reservoirs developed, such as the Sinian Dengying Formation, Carboniferous Huanglong Formation, Permian Maokou Formation, and the Triassic Leikoupo Formation^[10,12-18]. Since the gas discovery of Well Long 10 in a karst reservoir in 1957 in the Mid-Permian Makou Formation of the southern Sichuan Basin^[19], gas exploration in the formation has continued for more than 60 years, and it is still one of the main production layers of the southern Sichuan Basin. Three hundred and twenty-five fracture-karst reservoirs with total proved reserves of 852×10⁸ m³ had been discovered in the Maokou Formation in the southern Sichuan Basin by the year 2018, and the total production of the meritorious well of Zi 2 had reached 50.15×10⁸ m^3[17]. The proved reserves discovered at present in the southern Sichuan Basin are mainly distributed in the positive structures, while the slopes and negative structures with much larger areas have been drilled far less, particularly the negative structures. Several wells drilled in some synclines, such as Yunjin and Hejiang-Miaogaosi, have tapped high gas production flow, indicating large gas exploration potential in the slopes and synclines.

The reservoir types of the Maokou Formation in the southern Sichuan Basin are typically referred to as fractured or fractured-vuggy reservoirs, and the wells were typically drilled near or along the faults^{[19,20,21,22,23]}. Although the Maokou Formation in southern Sichuan has a long exploration history, its reservoir space types have not been classified elaborately, and the main reservoir types and distribution characteristics have also not been made clear, which has undoubtedly hindered the natural gas exploration of the formation. Guided by the exploration idea of searching for new reservoir types and plays out of the confine of faults and structures, the types and distribution pattern of the Maokou Formation reservoirs in the southern Sichuan Basin have been examined, in the hope to find new exploration plays in the slopes and synclines. Due to the scarcity of cores and imaging logging data of the Maokou Formation in the southern Sichuan Basin, the value range and shape of the conventional logging curves and core, drilling, and production data were used to identify the reservoir types of the Maokou Formation and sort out the logging responses of different types of reservoirs. Moreover, by interpreting log data comprehensively and analyzing drilling and formation testing horizons, the distribution characteristics and favorable natural gas exploration plays of the karst reservoirs in the Maokou Formation were studied and identified based on an analysis of comprehensive log interpretation, drilling, and testing data.

The Maokou Formation is missing in a large part of the Sichuan Basin. On the plane, the missing strata gradually increases from the southwest to the north and to the west of the basin (Fig. 1). Vertically, the Maokou Formation is divided into four members from the bottom up, namely, the first member (P₂m¹), the second member (P₂m²), the third member (P₂m³), and the fourth member (P₂m⁴). The first and second member can be divided into three submembers each, namely layer c, b, and a from the bottom up (Fig. 1). The fourth member is primarily distributed in the Yibin-Ya’an-Jiangyou and Shizhu areas in the southwestern and eastern parts of the basin. The third member, distributed widely, is only missing in the Wanzhou-Xuanhan- Wangcang area. The layer a of the second member of the formation is missing in the Wanxian-Dazhou area of the eastern Sichuan Basin, and the formation has been eroded to layer b of the Maokou Formation. Layer a of the formation is well preserved in the entire Sichuan Basin^[16,18-24]. Although different theories were proposed to explain the missing of the Maokou Formation, such as regional erosion due to drop of global sea level^{[18, 24-25]} and strata uplift during the Dongwu movement^{[17, 26-30]}, it is just because of the severe missing, karst reservoirs came about extensively in Maokou Formation of the Sichuan Basin.

The Leshan-Longnüsi area uplifted violently during the Caledonian movement, consequently causing a paleo-geomorphic pattern higher in the southwest and lower in the northeast of the basin. This type of paleo-geomorphic pattern changed little during the Hercynian movement, and mid-Permian deposited under such a paleo-geomorphic environment^[19]. The sedimentary environment during the Maokou period was ramp carbonate platform, where large areas of grain banks developed in the mid-west of the Sichuan Basin^[18]. Vertically, there are two periods of grain banks in the Maokou Formation, and the first one is layer b of the second member, and the second one is from the layer a of the second member to the third member^[18]. The second period of grain bank is superior to the first in continuity and thickness. The grain banks of the two periods provided a material basis for the karst reservoirs of the Maokou Formation^{[10, 18, 31]}.

The reservoir space of the Maokou Formation comprises pores and vugs, the fractures act as migration pathways, and the three combine into fracture-cavity systems. The systems vary greatly in scale, with some reaching 2 km, while others being only tens of meters across, with five orders of magnitude discrepancy between them^[23]. The reservoir rocks are primarily bioclastic limestone, micrite, and augen limestone, with a matrix porosity of less than 2% and permeability of less than 0.08×10^-3 μm^2[16].

The wells of the Maokou Formation in the southern Sichuan Basin have been drilled mainly over the last few decades, and core and imaging logging data are lacking. The conventional logging data of natural gamma (GR), density (DEN), acoustic interval transit time (AC), compensated neutron (CNL), dual lateral resistivity (R_lld, R_lls), and hole diameter (CAL) in combination with the drilling data, cores, thin sections, and productivity of 65 wells were used to identify the reservoir types of the Maokou Formation. The reservoir types were divided into four types, including fractured-vuggy, pore-vuggy, fractured, and fractured-cavity reservoir.

Fractures and vugs are highly developed in the fractured-vuggy reservoir, of which the logging curves response is characterized by “three lows and two highs,” including low GR (10-40 API), low ρ (2.55-2.75 g/cm³), medium-low R_lld and R_lls (100-3 000 Ω·m), high Δt (157.4-213.2 μs/m), and high ϕ_CNL (-1%-6%) (Fig. 2a), and these characteristics constitute the overlay response of the fractures and the vugs^[32]. The types of the conventional curves of the fractured-vuggy reservoir are mainly box- or bell-shaped, and the curves often reveal sawteeth or local jumping. The porosity of the fractured-vuggy reservoir ranges from 2% to 8%, and the phenomena of hole enlargement and lost circulations could often be found when the reservoirs were being drilled. Gas production in such a reservoir decreases gradually and can be sustained for a long time (Fig. 2b). Core photos show vugs and fractures are abundant, and some of the vugs are distributed irregularly in lines along the fractures (Fig. 3a and 3b). During uplifting of the strata, meteoric fresh water infiltrated into the limestone along vertical or high angle fractures, making fractures gradually grow bigger and finally become vugs due to dissolution. The dissolution vugs constitute the main reservoir space, while the fractures are the main seepage channels, and good configuration of vugs and the fractures is the key factor for keeping high and stable gas production.

Fractures are not well developed in the pore-vuggy reservoir, while the pores and vugs with diameters of less than 2 mm and 50 cm, respectively, are both well developed. The logging curves of this type of reservoir also have the characteristic of “three lows and two highs”, which is similar to that of the fractured-vuggy reservoir. However, the values of the conventional logging curves are different, including a low GR (10-30 API), low ρ (2.55-2.72 g/cm³), medium-low R_lld and R_lls (100-3000 Ω·m), high Δt (160.7-229.6 μs/m), and high ϕ_CNL (-0.5%-6%) (Fig. 4a). The curve responses of the reservoir changed collaboratively and slowly, and no sawteeth or local jumping were observed. The curves of AC and DEN were bell-shaped, while those of R_lld and R_lls were box-shaped or bell-shaped. The porosity of the pore-vuggy reservoir ranged from 2% to 8%, and hole enlargement and lost circulations could often be found when the reservoirs were drilled, which is similar to the fractured-vuggy reservoirs. The gas production decreases slowly and can be sustained for many years (Fig. 4b). The vugs constitute the main reservoir space and the fractures are not developed. The pores and vugs can be seen in the core (Fig. 3c and 3d). On image logging, the dissolved pores and vugs appear as low resistance dark patches several millimeter to tens of centimeter in diameter (Fig. 3e and 3f). Biological antrum pores and intercrystalline dissolution pores can be seen in thin cast sections (Fig. 3g). This type of reservoir experienced multiple periods of dissolution. Due to drop of sea level in the syngenetic stage or parasyngenetic stage, local parts of the grain banks exposed to the air in the depositional periods of P₂m²b, P₂m²a-P₂m³, and the primary pores went through dissolution, forming intragranular and intergranular pores. In the late depositional period of the Maokou Formation, the fresh water seeped down along existent pore-throats, dissolving the residual pores further, some parts were subjected to stronger dissolution due to more residual pores and stronger water current, forming dissolution pores and vugs of various sizes. The superposition of the two stages of karstification gave birth to the pore-vuggy reservoirs. The discovery of the pore-vuggy reservoir is of great significance for natural gas exploration in the southern Sichuan Basin, especially in the areas of slopes and synclines far from faults and structural highs.

The fractures are very well developed, whereas the pore and vugs are not well developed in this kind of reservoir. Previous studies show this kind of reservoir has slightly lower R_lld and R_lls, and has R_lld>R_lls when containing high angle fractures, and R_lls<R_lld when containing low angle fractures^{[33,34,35,36,37]}. Structural fractures, diagenetic fractures, and dissolution fractures comprise the reservoir space and pathways in this kind of reservoir. They have poor matrix physical properties, few secondary dissolution pores and vugs, and a porosity of less than 1% in general. They take on distinct sawteeth feature on log curves, have increase of Δt at the position of fractures (154.2 to 180.4 μs/m), little variation in ρ (2.73 to 2.69 g/cm³), R_lld and R_lls generally higher than 1 000 Ω·m, and small difference between R_lld and R_lls. In the fractured reservoir of the typical well B29, the R_lls is slightly higher than R_lld, indicating that there are low angle fractures (Fig. 5a). High angle fractures partially filled with calcite can be seen in the cores (Fig. 3h). Lost circulation can happen when such a reservoir is drilled. Production curves show they vary violently in gas production and have short production time (Fig. 5b).

This kind of reservoir has more cavities more than 50 cm in diameter^[2]. They feature enlargement of well caliper, dramatic increase of ϕ_CNL and Δt, and positive differences between R_lld and R_lls (R_lld>R_lls)^[34,35]. The dissolved caves in the Maokou Formation in the south Sichuan Basin are smaller, and the drilling break distance is generally less than 1 m. When the fractured-cavity reservoir is being drilled, the hole enlargement is very obvious, and the values of Δt and ϕ_CNL increase significantly to 170.6-236.2 μs/m and 0.5%-7% (more than 20% in individual wells), the ρ decreases suddenly from 2.6 to 2.1 g/cm³, the values of R_lld and R_lls are medium to low, ranging from 100 to 3 000 Ω·m, and box-shaped, with obvious positive difference (Fig. 6a). The value of GR increases if the cave is filled with mudstone, but would be around 10-40 API and would not change much if it is filled with calcite. Most of the caves in the core are filled with calcite and collapse breccia, and some of the caves reach 1-2 m in diameter (Fig. 3i), so lost circulation and drilling break are evident during drilling. They have stable production in a few years but big variation in production on the whole and long production time (Fig. 6b). The caves were formed by dissolution in local parts as fresh water seeped along the faults and fractures vertically and laterally, so they are strongly heterogenous and irregular in distribution.

The karst reservoirs in Maokou Formation are thinner (generally less than 10 m thick) each according to the comprehensive logging interpretation. They are mainly distributed in P₂m³, P₂m²a, and P₂m²b, and mainly distributed 0-100 m down from the top of the Maokou Formation. The reservoirs can be divided into two segments; the upper segment is P₂m³-P₂m²a 0-50 m down from the top of the Maokou Formation generally, and the lower segment is P₂m²b 50-100 m down from the top of the Maokou Formation. The upper reservoir segment is better developed than the lower one (Table 1 and Fig. 7), which is similar to those in the Maokou Formation in the Gaoshiti-Moxi area of the central Sichuan Basin^[38]. Two factors controlled the formation of the two segments of reservoirs. The first factor is the grain bank developed in P₂m²b and P₂m²a-P₂m³ two periods, which provided a material base for the formation of karst reservoirs^[10,18]. The second factor is the vertical zone of dissolution. The strata of P₂m³-P₂m²a are mainly in the vertical permeating belt, and P₂m²b is predominantly in the horizontal phreatic belt, and the two belts have strong control on the formation and development of the two segments of reservoirs.

The two segments of reservoirs both have high and stable production. The upper segment had a test production of 0.17- 477.90×10⁴ m³, and 39.13×10⁴ m³ on average, and cumulative gas production from 0 to 9.95×10⁸ m³ (1.44×10⁸m³ on average) according to the statistical results of 48 wells. According to the statistical results of 36 wells, the lower segment had a test production of 0.27-89.63×10⁴ m³, 15.11×10⁴ m³ on average, and cumulative gas production of 0 to 4.68×10⁸ m³ (0.77× 10⁸ m³ on average). Clearly, the upper segment has higher production than the lower one.

The karst reservoirs in the Maokou Formation have very poor lateral continuity and strong heterogeneity. The total thickness contours of the two segments of reservoir from log interpretation are shown in Fig. 8. The upper segment is 6 to 20 m thick, and is thicker in three areas, Yibin-Zigong-Weiyuan-Dazu area, the southwest area of Chongqing, and the southeast part of the Luzhou, with a thickness of 12 to 20 m. The lower segment is 3 to 12 m thick, and is thicker in Neijiang-Zigong-Luzhou area and Dazu-Luzhou area (6 to 12 m). The upper segment is better than the lower segment in terms of distribution area and thickness.

The Maokou Formation was deposited in a ramp carbonate platform, which is divided into shallow ramp in the west part and middle gentle ramp in the east part in southern Sichuan by the line Dazu-Luzhou^[18]. The grain banks in the area are 10 to 80 m thick, up to 60-80 m thick in the southeast areas of Luzhou and Weiyuan-Dazu, but thinner (< 40 m) in the Weiyuan-Yibin area^[18]. A comparison of the thickness contour of the reservoirs and the grain banks of the Maokou Formation in the southern Sichuan Basin (Fig. 1) shows they have good consistency, demonstrating that the grain banks have a control effect on the formation of the reservoirs.

There are four types of reservoirs in the Maokou Formation of the southern Sichuan Basin, namely, fractured-vuggy, pore-vuggy, fractured, and fractured-cavity reservoirs. Among them, the first two are the main ones and also reservoir types in high and stable production wells, as well as main exploration targets in the slopes and synclines of the southern Sichuan Basin.

The reservoirs in the Maokou Formation are generally less than 10 m thick each, and can be divided into an upper segment (layer a of the second member to the third member of Maokou Formation, P₂m²a-P₂m³) 0-50 m down from the top of the Maokou Formation and the lower segment (layer b of the second member of Maokou Formation, P₂m²b) 50-100 m down from the top of the Maokou Formation. The two reservoir segments are mainly controlled by grain banks, vertical karst zonation, and fractures.

The upper reservoir segment is better than the lower segment in terms of development degree, thickness, productivity and cumulative production. The reservoirs are thicker in several areas, including Neijiang-Zigong-Luzhou, Hebaochang Dazu, and the southern area of Luzhou. Therefore, the structural slopes or synclines in these areas are favorable areas for gas exploration.

Δt—acoustic interval transit time, μs/m;

d_h—hole diameter, cm;

ϕ_CNL—compensated neutron log porosity, %;

ρ—density, g/cm³;

GR—natural gamma, API;

R_lld—deep resistivity, Ω·m;

R_lls—shallow resistivity, Ω·m;

ϕ—porosity, %.