What are the types and distribution of oil and gas reservoirs in China?

Oil and gas reservoirs are composed of skeleton particles, matrix, cement and reticular pore system not occupied by consolidated parts. Pores in oil and gas reservoirs not only have the ability to store oil and gas, but also are the channels for oil and gas flow.

Rock types of oil and gas reservoirs in China mainly include clastic rocks, carbonate rocks, magmatic rocks and metamorphic rocks.

1. clastic rocks (conglomerate, sandstone, mudstone, pyroclastic rocks) The clastic rocks of oil and gas reservoirs are mainly continental clastic rocks.

1) The strata of various geological ages in China are distributed in time, and oil and gas are produced or displayed from Sinian to Paleozoic and Mesozoic, indicating that oil-bearing strata are widely distributed, but clastic reservoirs are mainly Mesozoic and Cenozoic, and some upper Paleozoic strata are also distributed.

Carboniferous clastic rocks are important oil and gas producing zones in China, producing oil in Tarim, Junggar and Bohai Bay basins, and gas in Sichuan and Ordos basins. Permian clastic rocks in Ordos basin and Su Qiao area of Jizhong depression produce natural gas and condensate oil, and Junggar basin and Erlian basin also produce oil. Triassic has oil and gas production in Ordos, Tarim, Junggar, Sichuan and Turpan-Hami basins; Jurassic produced oil in Turpan-Hami, Junggar, Tarim, Ordos, Qaidam, Sichuan, Bohai Bay, Songliao and other basins. The middle and lower Jurassic are coal-bearing strata, and natural gas is also produced in Taibei sag of Turpan-Hami basin. Cretaceous is the main oil-bearing area in Songliao and Erlian basins, and Tarim and Jiuquan basins also produce oil. Paleogene has oil production in Bohai Bay, Qaidam, Jiuquan, Junggar, Tarim, Beibu Gulf, Pearl River Estuary and East China Sea. The lacustrine and fluvial facies in Sebei and Salt Lake areas in the eastern depression of Qaidam Basin have formed quite large Quaternary gas fields.

2) The clastic reservoir sand bodies are continental basins, and the main sand bodies are rivers, deltas, fan deltas, nearshore underwater fans, lake bottom fans, beach bars and alluvial fans. There are some differences in oil-bearing sand bodies developed in different depressions. Daqing Oilfield in Songliao Basin has large river deltas, Shengtuo Oilfield and Huanghua Depression in Dongying Depression, with river deltas as the main oil-bearing sand bodies, followed by nearshore underwater fans, turbidity currents, beach bars and other sand bodies. Nanpu sag in the north of Huanghua depression mainly develops fan deltas and nearshore underwater fans, as well as rivers, deltas and beach bars. Fan delta and turbidite sand bodies are the main oil-bearing sand bodies in Liaohe Depression. Delta, turbidity current and beach bar are the main oil-bearing sand bodies in Dongpu Depression. The Jurassic in Taibei sag of Turpan-Hami basin is dominated by fan delta and braided river delta. Alluvial fan is the main oil-bearing sand body in Karamay Oilfield, Junggar Basin.

3) Clastic rock reservoirs can be divided into conglomerate, sandstone, siltstone and mudstone according to grain size, and also contain some pyroclastic rocks in composition (Table 2- 1).

(1) conglomerate.

Also known as coarse clastic rock, it is mainly composed of coarse clastic particles (gravels and breccias) larger than 2mm, and gravels and breccias are mainly composed of cuttings. The reservoirs composed of conglomerate are mainly alluvial conglomerate in the fourth member of Shahejie Formation in Zhanhua Depression of Jiyang Depression and turbidite conglomerate in the third member of Shahejie Formation in Dongpu Depression, as well as Triassic glutenite oil and gas fields in the northwest margin of Karamay Oilfield in Junggar Basin.

(2) sandstone.

Also known as middle clastic rock, it is mainly composed of sand grains, accounting for about half of sedimentary rocks, second only to clay rocks. More than 80% of clastic reservoirs in China are composed of sandstone.

The main composition of sandstone debris is chronological, followed by feldspar and various cuttings. According to grain size, it is divided into giant sandstone, coarse sandstone, medium sandstone and fine sandstone; According to the content of miscellaneous base, it can be divided into reticular sandstone and miscellaneous sandstone; According to the composition of detritus, it can be divided into timely sandstone, feldspar sandstone and lithic sandstone.

① Time-dependent sandstone: the time-dependent content of detrital components is more than 50%, and the content of feldspar and detritus is less than 25%, which can be divided into time-dependent sandstone (time-dependent sandstone accounts for more than 90%), feldspar time-dependent sandstone and lithic time-dependent sandstone; The complex type is feldspathic lithic sandstone. This kind of sandstone cement is mostly siliceous, followed by calcium, iron and glauconite. Typical timely sandstone and quartzite sandstone, which are mainly cemented in time, are almost all cemented in time, which is called sedimentary quartzite, and most of them are formed in marine environment or land-sea interaction environment, belonging to highly mature sandstone. Sandstones in Permian and Carboniferous gas-bearing basins in China belong to this type.

Table 2-1Rock types of clastic reservoirs

② Feldspar sandstone: timely content is less than 75%, feldspar content is more than 25%, and cuttings content is less than 25%. Feldspar composition is mainly potassium feldspar and acid plagioclase, and alkaline plagioclase is rare. The composition maturity and structure maturity of feldspathic sandstone are low, and the main transition type is lithic feldspathic sandstone. Typical feldspathic sandstone is mainly formed in active tectonic units and is the product of rapid transportation and deposition. Sandstone composed of more than 50% feldspar is called basement feldspar sandstone, which is mostly the product of accelerated accumulation in areas with strong tectonic activity. The sandstone reservoirs in Mesozoic and Cenozoic fault basins in China are mainly feldspathic sandstone (such as Bohai Bay oil and gas field and eastern Songliao basin), and feldspar dissolution will produce a large number of secondary pores in diagenetic stage.

③ Cuttings and sandstone: cuttings are more than 25%, feldspar is less than 25%, and the timely content is less than 75%. The cuttings are mainly composed of acidic extrusive rocks, phyllite, slate, mudstone, shale and various siliceous rock cuttings; Common cements are carbonate minerals and silica. Both composition maturity and structure maturity are low. With the increase of feldspar content, lithic sandstone gradually transits to feldspar sandstone. In Mesozoic and Cenozoic oil-bearing basins in China, such as Sichuan, Junggar and Turpan-Hami basins, lithic sandstone or feldspathic lithic sandstone generally exists. Due to the dissolution of unstable cuttings, a certain number of secondary pores can be formed, making it a better oil and gas reservoir.

(3) Siltstone.

Rock is mainly composed of silt particles, also known as fine clastic rock. It can also be divided into coarse siltstone and fine siltstone. Silt is dominated by time, followed by feldspar, often containing more muscovite debris, and the content of heavy minerals is higher than sandstone, which can reach 2% ~ 3%. Because it is rich in clay matrix, it is often transformed into silty mudstone and shale. Because it is often caused by suspension transportation-deposition, quartz silt is mostly angular, and mica debris is distributed along the river bed. According to different compositions, siltstone can be divided into feldspar siltstone and quartzite siltstone. Feldspar coarse siltstone in Mesozoic and Cenozoic continental sedimentary basins in China can be used as oil and gas reservoirs. Liuwen gas reservoir, Wendong gas field in Rudong Depression, Jilin Province, etc.

(4) Mudstone.

Buried mudstone is generally considered as a good caprock, but if mudstone fractures are developed and combined with source rocks and overlying strata, it can become a special oil and gas reservoir, such as mudstone gas reservoir in the west of Sichuan Basin and mudstone fractured reservoir in Qingshankou Formation in Gulong area of Songliao Basin.

(5) pyroclastic rocks.

Pyroclastic rocks are mainly composed of pyroclastic materials (>: 90%) and are a special kind of clastic rocks. It has the dual characteristics of volcanic rocks and clastic sedimentary rocks, and the material composition is mainly pyroclastic materials, which have a clastic rock-like structure and often coexist with normal clastic rocks, and can also be used as oil and gas reservoirs.

Pyroclastic rocks are distributed in the strata of all ages in China, especially in Mesozoic and Cenozoic continental sedimentary basins in eastern China. Some of them have good oil and gas display, such as Erlian Basin, and some small and medium-sized oil and gas fields have been obtained. Tuff and tuff can be used as oil and gas reservoirs, and the reservoir space is mainly secondary pores formed by dissolution of feldspar, unstable cuttings and even volcanic glass. Fine volcanic dust affects reservoir physical properties, and with the increase of volcanic dust content, reservoir physical properties become worse.

4) Features of clastic oil and gas reservoirs discovered in China ① Vertically, it can be divided into four sets of stratigraphic combinations: Carboniferous and Permian are chronosandstone, feldspathic chronosandstone and lithic feldspathic sandstone; Triassic is dominated by feldspathic sandstone and lithic feldspathic sandstone; The Jurassic and Cretaceous are dominated by feldspathic sandstone and chronological sandstone, and some lithic sandstone and pyroclastic rocks are also distributed. Tertiary and Quaternary are characterized by feldspathic sandstone, feldspathic sandstone and lithic feldspathic sandstone.

② The reservoir sand body of clastic reservoir is mainly river-delta sedimentary system, followed by fan-delta facies and lake bottom fan equal system.

③ The maturity of composition and structure is relatively low, which is related to the factors such as near-provenance, multi-provenance, rapid deposition and so on. The granularity of clastic reservoirs varies greatly, including conglomerate, gravelly sandstone, medium-coarse sandstone, fine sandstone, siltstone and argillaceous siltstone. Most of the gas-producing layers found in Mesozoic and Cenozoic are fine sandstone and siltstone, and most of them are coarse-grained sandstone in late Paleozoic. The sorting is medium, sub-circular-sub-angular, and the content of miscellaneous base in sandstone is generally less than 10%.

2. Carbonate reservoirs 1) are widely distributed in China. Sedimentary basins containing industrial carbonate reservoirs have been found in Sichuan, Bohai Bay, Ordos, Tarim, Pearl River Estuary, northern Jiangsu and Qaidam. From Proterozoic to Cenozoic, except for a few series, industrial oil and gas reservoirs have been discovered.

2) Characteristics of carbonate reservoirs (1) There are many oil and gas layers, mainly the Old Series, with the Paleozoic carbonate oil and gas layers first; Mesozoic and Proterozoic followed closely; The new generation is at the bottom.

(2) There are both marine and continental facies, and most of them are marine reservoirs. Proterozoic and Paleozoic carbonate reservoirs belong to marine sediments. The Mesozoic is dominated by marine facies, and a few are terrestrial facies; The Cenozoic is dominated by continental facies (lacustrine carbonate rocks).

(3) The diagenesis is intense, the primary pores disappear, and the reservoir space is dominated by secondary pores.

(4) Various pore types and complex reservoir structure. Pores, caves and fractures often exist in reservoirs, but they have different causes and distribution laws, and they are matched and superimposed with each other, forming various combination types and various reservoirs and permeability bodies.

(5) The physical properties of oil storage are poor, fractures play an important role in the generation and communication of oil and gas, and the reservoir is heterogeneous.

(6) It has the ability to generate oil and gas, and can form self-generating and self-storing oil and gas reservoirs. Some carbonate rocks are rich in organic matter, which can provide various pores during the formation and evolution of hydrocarbons.

3) Carbonate reservoir types Carbonate rocks can be divided into dolomite and limestone.

(1) Dolomite:

Dolomite is composed of granular dolomite (oolite, sand, algal blocks, algal sponge layer, core stone, etc.). ) and crystalline dolomite (mesomorphic dolomite, fine-grained dolomite and micrite dolomite), mainly distributed in Changxing Formation of Upper Permian and Sinian System of Lower Triassic in Sichuan. Such as Weiyuan Ziyang gas field, South Sichuan gas field, East Sichuan gas field and Southwest Sichuan gas field; There is also the gas field of Majiagou Formation of Lower Ordovician in Shaanxi-Gansu-Ningxia Basin.

(2) Limestone and dolomite:

Limestone consists of granular limestone (such as granular, oolitic, internal debris, algae, pellets, biological debris, crusts, etc.). ). The Tertiary Cladosporium limestone in Shengli Oilfield, Luoshu limestone in the middle of Sichuan Basin and Daanzhai crust limestone in China all belong to this type. Secondly, it also includes crystalline limestone and crystalline dolomite limestone.

Limestone and dolomite oil and gas fields include related oil fields in southern Sichuan, eastern Sichuan and western Sichuan in Sichuan Basin, oil and gas fields in Huanghua Depression in central Sichuan Basin, Fangdawang Gas Field and Daanzhai Oil Field in Jiyang Depression. Its characteristics are shown in Table 2-2.

Table 2-2 Carbonate Reservoir and Rock Type

3. The special rock reservoirs of magmatic rocks and metamorphic rocks mainly refer to magmatic rocks, such as diabase, basalt, andesite, rhyolite and vein rocks, and metamorphic rocks, such as phyllite, slate, gneiss, migmatite, granulite and metamorphic sandstone. In recent years, in the process of oil and gas exploration in Junggar, Bohai Bay and other basins, northern Jiangsu, Sichuan Ya 'an and other places, oil and gas accumulation zones have also been found in metamorphic rocks and volcanic rocks, as shown in Table 2-3.