As a hotspot of oil and gas exploration, exploration and development of metamorphic rock has drawn high attention from petroleum geologists^[1,2]. They have realized that the study of reservoir is very important for the exploration and development of metamorphic rock oil and gas. In foreign countries, the protolithic characteristics^[3], structural bedding^[4], and epizonal metamorphism of metamorphic rock^[5] have been studied, but metamorphic reservoirs and reservoir formation are seldom involved. In China, metamorphic reservoirs have been found successively in Liaohe Depression, Dongying Sag and Bei’er Fault Depression of Hailar Basin in Bohai Bay Basin. Archean metamorphic buried hill oil reservoir rocks in Liaohe Depression are migmatite, biotite plagioclase gneiss and cataclastic rocks. Reservoir formation controlling factors^[6], reservoir characteristics^[7,8], comprehensive evaluation, and fracture characterization^[9] and reservoir formation conditions of the oil reservoir^[10] have been studied. The buried hill reservoir rocks in Dongying Sag mainly include Archaean gneiss and granulite^[11]; the parametamorphic reservoirs of Budate Group in Bell Fault Depression are mainly cataclastic andesite tuff, altered volcanic rock, unequal-grained metamorphic sandstone and collapsed breccia^[12,13]. These metamorphic reservoirs are mainly orthometamorphic rocks with igneous rocks as original rocks, and no systematic study on the characteristics, formation and distribution of parametamorphic reservoirs has been reported.

In recent years, great breakthroughs have been made in the exploration of basement weathering crust in Qaidam Basin, foretelling broad prospects. In the early stage, focusing on the high position of basement buried hill, the weathering crust oil reservoir of metamorphic rock was discovered in Mabei area, piedmont of Qilian Mountains^[14], the weathering crust reservoir of granite was found in Kunbei area, piedmont of Kunlun Mountains^[15], and large gas reservoirs of weathering crust of granite and granitic gneiss were discovered in Dongping area, piedmont of Altun Mountains^[16]. Besides distributed in the high position of the ancient buried hill, thickness of these reservoirs are controlled by the thickness of the weathering crust. Recently, a new parametamorphic rock gas reservoir was discovered in the west of Pingdong Fault (hereinafter referred to as Pingxi area). Compared with the above-mentioned basement reservoirs, the reservoir rock types are different greatly, and it is located in structural low, in the present slope-sag area. Vertically, it has multiple gas-bearing strata of big cumulative thickness. The daily average gas production per well is more than 2×10⁴ m³. It can be seen that the newly discovered parametamorphic reservoirs are expected to expand the field of natural gas exploration to the deep strata in the slope-sag area of the basin. Therefore, its reservoir characteristics, formation mechanism and distribution law need to be studied urgently.

Pingxi area is mainly located in Pingxi slope area, west of Pingdong fault in Qaidam basin, and part of Dongping uplift. It is a south-dipping paleoslope structure controlled by NS strike faults (Fig. 1). Since Mesozoic era, the eastern piedmont of Altun Mountains has experienced three tectonic evolution stages: late Yanshanian movement, early Himalayan movement and middle-late Himalayan movement^[17]. From west to east, the structural framework of Pingxi sag, Pingxi slope, Dongping uplift, Niubei slope and Pingdong sag has been formed, with strong structural inheritance and abundant small faults. The basement of Pingxi area is mainly composed of Proterozoic-Paleozoic metamorphic rock and granite. Zircon U-Pb dating shows that the age of biotite granite at the very bottom is 406.9±4.4 Ma, that is to say it was formed in the Early Devonian^[18]. Ordovician-Silurian parametamorphic rocks are distributed above Cambrian gneiss in the lower part of the basement. They are composed of calc-schist, crystalline limestone and slate with thickness of 300-600 m, 150-250 m and 0-100 m, respectively. Above the parametamorphic rocks are residual paleo-soil layers or upper Jurassic series composed of gray, brown-gray siltstone and purple mudstone of 10-130 m thick of delta and lacustrine facies. Above them are Cenozoic Paleogene Lulehe Formation and Lower Ganchaigou Formation. The lower part of the Lulehe Formation is dominated by thick brown conglomerate and grayish-white argillaceous gypsum, while the upper part is dominated by brown mudstone with argillaceous siltstone of alluvial fan and alluvial plain facies with a thickness of 1 000-1 500 m. The lower part of the Lower Ganchaigou Formation is dominated by gray-brown gravel-bearing fine sandstone and medium- fine conglomerate, representing braided river facies; the upper part is dominated by brown mudstone with gray siltstone of braided fluvial, braided delta and lacustrine facies with a thickness of 1 200-1 600 m.

Core, thin section, scanning electron microscopy and X-ray diffraction analysis confirmed that there are three types of parametamorhpic rock reservoirs in Pingxi area. The first type is the Silurian slate, gray or gray-green, with slab structure and palimpsest texture, possessing residual metamorphic structure, and residual quartz and feldspar in directional arrangement (Fig. 2). The main mineral components are clay and quartz, the two combined account for more than 60%; the calcite accounts for 5% to 16%; the feldspar group accounts for 2% to 3%, largely albite. The other mineral components include a small amount of pyroxene (mainly from the original rock), anhydrite, barite, and pyrite (Table 1). The second type is the Ordovician crystalline limestone, with low metamorphism degree and some original rock features. The crystalline limestone is the main rock type in the Pingxi area, gray, grayish white or greenish gray, with massive structure and high content of crystalline calcite in directional arrangement (Fig. 2). The mineral composition is chiefly calcite, accounting for 45% to 60%, and the content of dolomite is low; quartz and clay minerals each account for 10% to 25%; the rest are a small amount of albite, siderite, barite and anhydrite (Table 1). The third type is the Ordovician mica- quartz-bearing calc-schist of dark gray and grayish white with slab structure; in it are some calcite and sericite minerals in directional arrangement. The biotite and calcite have rich cleavages (Fig. 2). Calcite and clay are the major mineral components, combined accounting for 42% to 72%; the second is quartz, accounting for 20% to 40%; the proportion of feldspar is 0.5% to 12.0%, mainly albite. The rest components in small quantity include analcime, barite, and anhydrite (Table 1). In summary, the three types of parametamorhpic rock reservoirs in Pingxi area can be told from each other clearly. Slab structure is common in slate, cleavage is very developed in calc-schist, and crystalline limestone has non-obvious structure due to high content of crystalline calcite with high plasticity. The distribution of granular and flaky minerals in parametamorhpic rocks has a good correlation with the degree of metamorphism. The calc-schist has the highest metamorphism degree and the highest degree of mineral directional arrangement, crystalline limestone is the second, and slate is the last.

The analysis of rock mineral composition and combination characteristics of metamorphic rocks can restore their original rock types, and the original rocks of the metamorphic rocks such as slate, schist, gneiss, quartzite and marble are mostly sedimentary rocks^[19,20]. The rock types of parametamorhpic rocks in Pingxi area are mainly slate, crystalline limestone and calc-schist, the original rock of them is Ordovician-Siludian sedimentary rock. X-ray diffraction analysis of the whole rock indicates that the slate is mainly composed of quartz and clay (aluminum-rich series) (Table 1), with residual quartz and feldspar visible in the thin-section (Fig. 2b), so the original rock of slate is mudstone, silty mudstone, siltstone or fine sandstone. The crystalline limestone is made up of calcite (rich calcium series), quartz and clay largely in close contents (Table 1), indicating that the original rocks are mainly silty limestone, silt-bearing micrite and argillaceous limestone. The main minerals of calc-schist are clay, calcite and quartz close in content (Table 1), with calcite and sericite visible in the thin-section (Fig. 2h), which shows that the original rocks are mainly lime siltstone, lime mudstone and argillaceous limestone, and silty limestone. The granite around the parametamorphic rocks is the product of lava cooling formed in early Devonian, so the original rock must be formed in the Ordovician-Silurian period before. The primary rocks of the parametamorphic rocks in the Pingxi area are mainly carbonate, with some clastics and clay rocks, indicating they were formed in the Ordovician-Silurian marine sedimentary environment.

The epizonal metamorphic reservoirs in the Pingxi area have kinds of reservoir spaces, which can be divided into three types and six subtypes according to their shape, basic characteristics and formation mechanism. The first major type is fracture, which can be subdivided into three sub- types, structural fracture, dissolution fracture and weathering fracture. The structural fractures include high angle, low angle and multi-angle ones. High angle fractures can be seen in all the three kinds of parametamorhpic rocks, with the angle of generally greater than 60°, and are formed by extrusion shearing. These fractures are regular in shape, straight in fracture wall, strong in cutting force and bearing capacity, large in scale, and not easy to close (Fig. 3a). They improve the reservoir performance of the rock and are the most efficient storage space. Formed by the slip-squeezing stress, low-angle fractures are less than 20° in angle or nearly parallel, regular and straight in shape, and clear in groups. Commonly seen in crystalline limestone and calc-schist, they often intersect with high-angle fractures (Figs. 2h and 3b). Multi-angle fractures are related to structural extrusion, and most developed in stress concentration release zones, generally in the form of grids, breccia, and disordered groups, cross-cutting, and different in opening and extension from each other. The multi-angle fractures often exist in slate and calc-schist (Fig. 3c). The dissolution fractures are generally curved, harbor-shaped, with long extension. They are the product of dissolution enlargement along fractures, controlled by the weathering and leaching intensity in development degree, and more common in crystalline limestone and calc-schist (Fig. 3d). The weathering fractures are cleavage seam (slice seam) formed by physical weathering and fracturing of the rock along lithological planes. Often in linear and funnel shapes, they are most common in calc-schist, followed by slate (Fig. 3e). The second major type is dissolved pore, which can be subdivided into two sub-types, dissolved pore and dissolved cavity. The dissolved pore with diameter of less than 2 mm can be found in all the three types of parametamorphic rocks, and are large in number and irregular in shape (Fig. 3f). The dissolved cavities with diameter greater than 2 mm, often occur in calcareous schist, along fractures or isolated, and small in number (Fig. 3g). The development of dissolved pores and cavities are controlled by weathering and leaching. The weathering product is mainly clay, and calcium is easily dissolved and taken away by water. The third major type is nano-scale intercrystalline pores, which are angular, tiny in size, visible under scanning electron microscopy and exist in crystalline limestone locally. Since the original rock of the crystalline limestone is dominated by mud-silt-grade rocks and the calcite crystals are small, the intercrystalline pores formed are subsequently small (Fig. 3h).

2.4.1. Physical properties

The core magnetic resonance osmosis test showed that the slate has the best physical properties, with a porosity of 5.0%- 6.3%, 5.5% on average, and calculated permeability of (0.2- 1.4)×10³ μm², 0.8×10³ μm² on average. The crystalline limestone takes the second place, with a porosity of 2.6% to 7.2%, on average of 4.4%, and calculated permeability of (0.01 to 1.06)×10³ μm², on average of 0.3×10³ μm². Calc-schist has the worst physical property, for example, Well Dp172, calc- schist has a porosity of 1.4% to 6.2%, on average 3.5%, calculated permeability of (0.01 to 0.10)×10³ μm², on average 0.06×10³ μm²; and calc-schist in Well Dp17, has a porosity of 1.2% to 4.3%, on average 2.8%, calculated permeability of (0.01 to 0.33)×10³ μm², on average of 0.05×10³ μm² (Table 2). According to the classification criterion of volcanic reservoirs in the petroleum industry^[21], the parametamorhpic rocks in the Pingxi area belong to type IV reservoir, that is, ultra-low porosity and low permeability. The fracture is an important kind of reservoir space for parametamorhpic rocks in Pingxi area, but the test analysis of core samples can only reflect the local physical properties of the reservoir, and it is difficult to reflect the impact of the fracture on the overall physical properties of the reservoir. The existence of fracture would improve overall physical properties of the metamorphic rock reservoirs significantly.

2.4.2. Pore throat

Micro-CT analysis of core samples shows that all the three types of parametamorhpic rock reservoirs have abundant pores and throats, but the pores and throats are different widely in quantity, volume and radius. The slate with few fractures has pore throats small in number, volume, and radius. The pore throats are mainly micro-pores and homogenously distributed (Fig. 4a). Crystalline limestone and calc-schist with abundant fractures have pore throats concentrated along fractures, which are larger in number, volume and radius than those in the slate (Fig. 4b, 4c). It can be seen that the development degree of fracture has a strong impact on the formation of pore throats in the latter two kinds of parametamorhpic rock reservoirs. In addition, the type of fracture also has a strong impact on the pore throats. Structural fractures with big aperture and better connectivity would give rise to more pores of larger size. Cleavage fractures with small aperture would result in a lot of pores too, but these pores are smaller in size (Fig. 4b, 4c).

The parametamorphic rock reservoirs in Pingxi area are mainly formed by metamorphism, weathering, structural fragmentation and dissolution.

Previous studies on metamorphism mainly focused on the types of metamorphic characteristic and metamorphic facies^[20], but did not cover the influence of metamorphism on reservoirs. In Pingxi area, metamorphism not only gives rise to three types of metamorphic rock reservoirs: slate, crystalline limestone and calc-schist, but also has a great influence on the reservoir properties. Calc-schist is highly metamorphic, with flaky and columnar minerals and good stratification (Fig. 2g-2i), which is conducive to the generation of cleavage fractures by weathering and thus to expand reservoir space (Fig. 3e). The crystalline limestone with high metamorphic degree has calcite in directional arrangement, which is favorable for the formation of dissolution pores along the layer (Fig. 5a). The crystalline limestone with low metamorphic degree would retain more protolithic characteristics and develop intergranular pores (Fig. 3h). Metamorphism has a great influence on the reservoir performance of calc-schist. Its degree of action is positively correlated with the reservoir physical properties. For example, the measured porosity of calc-schist cores from wells Dp17 and Dp172 tends to increase with the increase of burial depth (Fig. 6). Besides, crystalline limestone is buried shallower than calcareous schist, and has more matrix pore in original rock than calc-schist. However, the base value of porosity peak line by logging of crystalline limestone in Well Dp172 and Well Dp174 is generally lower than that of calcareous schist (Fig. 7). This also shows that the degree of metamorphism is positively correlated with reservoir physical properties, and high degree of metamorphism would result in good reservoir physical properties. Therefore, metamorphism plays a constructive role in deep buried calc-schist, so parametamorphic reservoirs can also occur in deep strata.

Weathering of rocks is mainly influenced by paleoclimate, tectonic movement and paleogeomorphology, among which tectonic movement controls its duration and paleogeomorphology influences its strength. Influenced by multi-stage basement uplift, the parametamorphic rocks in Pingxi area had long exposed to the surface before Paleogene sedimentation, and were subjected to tens of millions of years of weathering of high intensity and deep depth. There are two types of weathering in the shallow metamorphic rocks: physical and chemical. Depending on rock type and temperature difference in strength, physical weathering lasted for a long time and resulted in a lot of cleavage and crushing fractures with poor directionality in the parametamorphic rocks, and consequently fragmentation of the rock along cleavage planes or disorder crushing (Fig. 5b). Unsurprisingly, coring recovery is low. Statistical results show that coring recovery of Well Dp17 is less than 50%. Chemical weathering is affected by seasonal atmospheric precipitation, and occurs intermittently along cracks to form dissolution pores. The intensity of physical and chemical weathering often decreases with the increase of rock burial depth, and the change of terrain will lead to the difference of weathering intensity^[21,22]. Based on the interpretation and analysis of the seismic flattening section, the pre-Paleogene sedimentary paleogeomorphology in Dongping area was restored, excluding the influence of stratigraphic dip, compaction and denudation. The seismic profile shows that the thickness of the Paleogene Lulehe Formation between the Hongnan and Pingbei faults (Well Dp5-Well Dp8 area) was obviously thicker, and the residual Jurassic under the Lulehe Formation and the coaxial reflection characteristics of the basement are obviously different from those on both sides (Fig. 8). The paleo-geomorphic map also shows that the terrain was high in the north and low in the south, the north was the ancient uplift area, the southeast was the ancient slope area, and the southwestern Pingxi area was in the ancient depression area low in terrain (Fig. 9). Generally speaking, the special paleogeomorphology of depression in southwestern Pingxi area was beneficial not only to the preservation of epizonal metamorphic rock reservoirs but also to their long-term weathering.

The weathering has great influence on parametamorphic reservoirs of slate and crystalline limestone, and its intensity is positively correlated with reservoir physical properties. For the same rock type, as weathering intensity decreases with the increase of depth, the physical properties get poorer. For example, the measured porosities of crystalline limestone cores from Well Dp17 (Fig. 6a) and slate cores from Well Dp172 (Fig. 6b) decrease with the increase of depth. The baselines of peak porosity calculated for Well Dp172 slate and Well Dp17 crystalline limestone also show the same regularity (Fig. 7).

Structural fragmentation formed structural fractures in parametamorphic reservoirs of Pingxi area, which happened in two stages. The tectonic cataclasis of the first stage in Cretaceous mainly formed low angle fractures, and the second stage in Neogene produced high-angle fractures and micro-multi- angle fractures. Fracture development stages have a significant impact on reservoir formation. Core data and scanning electron microscopy analysis show that the low angle fractures formed in the first stage are mostly filled with little reservoir space left; the high angle and multi angle fractures produced by tectonic cataclasis in the second stage can be effective reservoir space (Figs. 3c and 4b). The extent and scope of the second stage of structural fragmentation are greater than those of the first stage, and the fractures generally cut or obliquely intersect rock layers, causing obvious dislocation or displacement (Fig. 5c). The development degree of structural fractures is related to rock type, the calc-schist is liable to form a large number of high-angle fractures (Fig. 5c), while the slate and crystalline limestone have less structural fractures (Figs. 2a and 3a).

Dissolution is a main cause of the formation of matrix pores. The parametamorphic metamorphic rocks have high content of unstable minerals, for example dark minerals in slate and aluminosilicates, calcite in crystalline limestone, dark minerals in calcareous schist and calcite are soluble minerals, which can be leached and dissolved by atmospheric fresh water to form pores. The development of dissolution pores is controlled by quality and quantity of fractures. Structural fractures and weathering cleavage fractures can provide migration channels for dissolved mineral elements. Dissolution would produce mainly two types of reservoir space, dissolution fracture and dissolution pore, cavity and micropore. The dissolution fractures dissolve and enlarge reservoir space along structural or cleavage fractures (Fig. 3d); the dissolution pores, cavities (Fig. 3f, 3g) and microspores (Fig. 5d), generally occur near fractures, forming a good pore-fracture reservoir system (Fig. 4b, 4c).

Based on the types of original rocks, magma intrusion period, tectonic movement and basement uplift, reservoir formation models in five stages in Pingxi area have been established. The first stage is the protolithic formation stage. On the Cambrian granite basement, three lithologic segments, Ordovician micritic limestone interbedded with mudstone, micritic limestone, and Silurian mudstone interbedded with sandstone, with largely primary pores. The second stage is the formation stage of parametamorphic rocks in early Devonian. At this time, the lava began to move and intrude upward along the Pingxi fault formed by tectonic compression, which metamorphized the Cambrian granite into gneiss, Ordovician marine sedimentary rocks into calc-schist and crystalline limestone, and Silurian clastic rocks into slate, forming the three types of reservoirs with bedding development degree and stratification turning poorer upward. Pingxi fault is large in fault throw, and the footwall strata are well preserved, while the upper Silurian and Ordovician strata in the hanging wall are denuded. The third stage is weathering and denudation stage. From Carboniferous to Triassic, the uplift of the basin basement resulted in long-term weathering and denudation and dissolution of the epizonal metamorphic rocks, which greatly improved the reservoir performance of slate and crystalline limestone. Most of the slate in the footwall of the fault was denuded and only small part remained locally, and a large number of dissolution pores were formed along weathering fractures in the upper and middle parts of the fault; the lower Ordovician and Cambrian strata developed in the hanging wall. The fourth stage is the formation stage of low-angle fractures, which was caused largely by the structural fragmentation in the middle Cretaceous, forming a large number of low-angle fractures. Most of the weathering fractures, solution pores and low angle fractures caused by weathering and dissolution were filled in the late stage, resulting in the loss of reservoir space. In the footwall of the fault, the distribution range of slate became smaller, and the Jurassic formation covered on the top of the slate, while in the hanging wall of the fault, the Ordovician was denuded, leaving the Jurassic and Cambrian formations. The fifth stage is the formation of high-angle fractures. In the early Neogene, a large number of high-angle and micro-multi-angle fractures were formed by tectonic shear compression, and a large number of dissolved pores were produced by dissolution. In the footwall of the fault, Jurassic and Paleogene covered crystalline limestone and slate, in the hanging wall, the Jurassic was denuded, and the Paleogene covered directly on granite or gneiss.

The parametamorphic rocks in Pingxi area have three lithologic segments from top to bottom, namely slate, crystalline limestone and calc-schist beneath the stable pale-soil layer of 10-30 m thick. The first lithology section is mainly composed of thick crystalline limestone with thin calc-schist and locally distributed slate, mainly in the middle and upper Ordovician. The crystalline limestone is 50-210 m thick, the calc-schist is 5-20 m thick, and the slate is about 80 m thick, which quickly pinch out to both sides. The second lithology section is calc-schist intercalated crystalline limestone, mainly located in the middle Ordovician. The calc-schist is 50-140 m thick and crystalline limestone 40-70 m thick. The third lithology section is mainly calc-schist of more than 100 m thick (Fig. 10), which is mainly located in the lower Ordovician. From the vertical high value distribution of total hydrocarbon curve of exploration wells, the first lithologic section has the most oil and gas shows and is obviously better than the second lithologic section, but their lateral connectivity is poor (Fig. 10). This shows that the reservoirs are highly heterogeneous longitudinally, which is mainly affected by the following factors: first of all, there are many types of original rocks of parametamorphic rocks, including clastic rocks, clay rocks and carbonate rocks. The longitudinal lithofacies change rapidly, so there are many types of parametamorphic rocks and the longitudinal lithofacies change quickly. Secondly, the degree of metamorphism from top to bottom of parametamorphic rock reservoirs decreases and the weathering intensity weakens. Finally, the heterogeneity of parametamorphic rock reservoirs is also greatly affected by the differences in fracture types and development degree. The first lithologic section was located in the upper part of the basement, with shallower burial depth, so it was greatly affected by weathering. The modification of weathering and leaching makes the reservoir physical properties of this section much better than the second lithologic section on the whole. Therefore, the first lithologic section should be the main target formation for current exploration, which has been proved by exploration, 4338-4 351 m (lithologic section 1) in Well Dp17 was tested an industrial gas flow of 5.1×10⁴ m³. Represented by Well Dp171 and 174, the third lithologic section has good full hydrocarbon shows in the deep part, which indicates that parametamorphic reservoirs can still exist in the deep part. This is because the third lithologic section has the deepest burial depth, so it was influenced by metamorphism most greatly, with the highest degree of metamorphism. The bedding structure formed by metamorphism plays a constructive role to the reservoir. Therefore, the exploration of parametamorphic reservoirs can break away from the restriction of burial depth, as better reservoirs can also exist in the deep part. The third lithologic section is also an important target for the next step exploration. The 4 778-4 788 m (lithologic section 3) in Well Dp171 was tested and obtained a daily gas production of 4.9×10⁴ m³.

Paleogeomorphology controls the plane distribution of the parametamorphic rock. In the paleo-uplift area high in terrain, the parametamorphic rocks are mostly denuded; while in Paleosag and slope areas with low terrain, the parametamorphic rock are preserved (Fig. 7). The parametamorphic rock reservoirs in Pingxi area have different root mean square amplitude responses. The comprehensive analysis of log interpretation and seismic data shows that the slate at the top of Well Dp172 is of medium and weak amplitude; crystalline limestone of Well Dp17 is moderate and weak amplitude; and calc-schist is strong amplitude (Figs. 11 and 12). The root mean square amplitude attribute between Hongnan fault and Pingxi fault is strong on the whole, but weak on outside of this area. Based on the plane amplitude attribute of root mean square, using the comprehensive calibration of core and logging data, the plane distribution characteristics of three kinds of metamorphic rocks in Pingxi area were analyzed. Parametamorphic rocks mainly distribute between Hongnan and Pingxi faults, and slate is distributed locally in Well Dp172 and Dp8 areas; large area of crystalline limestone is distributed around the slate, covering Well Dp8 and Dp17 area; calc-schist exists in the east of Well Dp173 and Dp8 and west of Well Dp17. On the plane, slate distribution area is smaller, calc-schist distribution area is wider, and crystalline limestone distribution area is the widest (Fig. 13). Calc-schist and crystalline limestone are also the main types of parametamorphic rock reservoirs in Pingxi area. To date, industrial gas flow has been obtained from crystalline limestone and calc-schist in Dp17 well area. Therefore, the favorable areas for the next exploration of parametamorphic rocks are the development areas of calc-schist and crystalline limestone in the east side of Well Dp173 and southwest of Well Dp17.

The rock types of parametamorphic rock reservoirs in Pingxi area of Qaidam Basin include mainly slate, crystalline limestone and calc-schist. The original rocks are marine sedimentary rocks of Silurian-Ordovician. There are 3 types and 6 sub-types of reservoir space in the reservoirs, the first type is fracture, including structural fracture, dissolution fracture and weathering cleavage fracture; the second type is dissolution pore, including dissolution pore and dissolution cavity; the third type is nano-scale intergranular pore. Parametamorphic rocks are extra-low porosity and ultra-low permeability reservoirs. The development degree and types of fractures control the pore throat distribution of parametamorphic rock reservoirs. Pore throats around fractures are well developed. Structural fractures are more likely to produce large pore throat radius reservoirs than schistose fractures.

Metamorphic, weathering, tectonic fragmentation and dissolution are the keys to the formation of metamorphic reservoirs. The metamorphism degree of calc-schist is positively correlated with reservoir physical properties. The stronger the weathering intensity of slate and crystalline limestone is, the better the reservoir physical properties are. Tectonic fragmentation happened in two stages and formed many types of structural fractures. Dissolution fractures and pores are produced by dissolution, which is the main reason behind the increase of matrix porosity. The formation of parametamorphic rock reservoirs experienced five main stages: original rock, parametamorphic rock, weathering and denudation, low angle fracture and high angle fracture.

The parametamorphic rock reservoirs are influenced by rock type, weathering intensity and metamorphic degree vertically, and strong in heterogeneity. The first and third lithologic segments are the main target strata for next exploration. The parametamorphic rock is sandwiched in the paleo-depression area between Hongnan fault and Pingxi fault on the plane. The calc-schist and crystalline limestone are widespread in the east of Well Dp173 and southwest of Well Dp17, which are the next favorable exploration zones.