GREAT OIL EXPLORATION POTENTIAL IN SICHUAN BASIN
By Wang Yujun / CNOOC Industrial Company Gaodianbei Base Branch Co.
Sichuan basin is one of large basins in China continent with a total area up to 190,000Km2 and a thickest sedimentary rock over 20,000m where marine sedimentation goes from Sinian system to middle and lower Triassic series; continental sedimentation goes from upper Triassic series to Tertiary system. Through 50 years of exploration, over 20 oil fields and oil-bearing structures have been found in Sichuan basin, including such oil-bearing formations as Xujiahe formation, Da’anzhai formation, Lianggaoshan formation and Shaximiao formation, carbonatite reservoirs, sandstone reservoirs as well as hydrocarbon-bearing anticlines, synclines and inclines.
8 Exploration Highlights in Sichuan Basin
Increasing oil fields and enlarging oildoms -- In 1958, great attention was attached to the exploration in central Sichuan block where over 20,000 Km2 of oil fields were found soon after the high production of Nanchong, Longnvsi and Penglai structures, including Guihua, Yingshan, Guang’an, Yilichang, Hechuan and Luodu etc.
From the 1960’s to early 2000’s, reservoirs have been found in Jinhua, Qiulin, BJC, Zhongtaishan and Shilongchang (oil-gas fields) at the north of Penglai and Guihua oil fields, Gongshanmiao, Shuijiacao and Longgang etc. at the west and north of Yingshan oil field, Tongnan and Anyue oil-gas fileds at the southwest of Hechuan oil field. Oildom in Sichuan has been enlarged to over 30,000 Km2.
Oil fields distributed in anticline, syncline and incline -- The explored Nanchong, Longnvsi, Yingshan and Guang’an etc. are anticline structures. Oil fields have been found in syncline between declined incline and anticline structure through field extension and wide open exploration: Guihua oil field at the east of Penglai oil field is on north incline; Liaojia Shiba oil field is in the syncline between Longnvsi and Wenchangzhai anticlines; Lianchi oil field is found in west cap of Nanchong oil field.
The wide distribution of oil fields shows that the accumulation of oil and gas may be determined not only by structure, but also stratum, lithology and fracture etc., which is good for the exploration of all kinds of reservoirs.
Low/mid- porosity and low/mid-permeability reservoirs -- It is generally believed by experts through drilling, test and analysis that Xujiahe, Da’anzhai, Lianggaoshan and Shaximiao formations are low-porosity and low-permeability reservoirs, but there might be some exceptions. There were high production oil and gas flows through exploration in other formations where sandstone and shell limestone reservoirs with good porosity and permeability were found, so specific analysis for the determination of reservoirs should be made for the exploration in Sichuan zone, for example, Xi 22 well in northwest Nanchong structure has a sandstone porosity of 12% and permeability larger than 10 mdarcy through coring at Lianggaoshan formation and gets 29~57 tons of crude oil in DST; Da’anzhai formation of Guihua oil field has pure shell limestone, thick reservoir, good physical property and high production.
Three thrust belts, key exploration areas in Sichuan basin -- Longmen Mountain thrust belt in northwest of Sichuan basin, Daba Mountain thrust belt in the north and Huaying Mountain thrust belt in the east. In hundreds of kilometers of thrust faults, upthrown block exposes the Palaeozoic strata and the downdropped block reserves the strata of Jurassic system and Triassic system. That will form a thrust fault to block reservoir. Some useful traps have been found in three thrust belts, which will be the important exploration targets (Fig 1).
Wide synclines in east, south and west Sichuan useful for forming Jurassic and Triassic reservoirs --Mesonzoic and Palaeozoic reservoirs have been found in high-angle anticline in east Sichuan and low-even anticline in south Sichuan. Among anticlines, it is the thick Jurassic system with reservoirs, hydrocarbon source rocks and cap rocks. Deep oil and gas may also transfer to Jurassic system along the fault, and then form the Jurassic reservoir. Oil flow has been explored in Luzhou syncline, and gas rush in syncline of east Sichuan. Tons of crude oil has been explored in Xujiahe formation of Qionglai gas well structure in west Sichuan. So oil exploration in Sichuan will be expanded to west, south and east Sichuan (Fig 2).
Precise seismic data of big or small faults and structure deformation useful for the success of exploration --During the 1950’s to 1970’s, seismic technology was still under developed. Seismic profiles lacked of correctness and definition. Since the reform and opening-up, seismic technology develops rapidly; now, 2D and 3D seismic profiles are available; clear curves of big or small faults and structures are good for the determination of fault-fracture reservoir; some profiles with river sands clearly distributed on are useful for the prediction of reservoir. Fracture prediction technology (seislog) developed by Beijing Exploration & Development Research Institute shows good results and will be significant to boost the success of exploration.
Fracture analysis and prediction will speed up the oil exploration in Sichuan. -- SPA and Southwest Oil & Gas Branch Company have found a set of effective exploration methods in long-term experience of exploring fractured reservoirs, e.g. three AT (at high point, at structural saddle and at cap) and three ALONG (along axis, along fault and along twist) for the exploration of hydrocarbon reservoirs; prospecting well at/near small fault (fracture) and away from big fault (a certain distance from big fault). If explore along axis, attention should be attached both on axis of anticline and axis of syncline. In the prediction of shell limestone reservoir of Da’anzhai formation, importance is put on the research of sedimentary environment and the aim is to find high-energy belt.
40-year continuous exploration in Guihua oil field in abundant Sichuan – Since the exploration of Peng 40 well in 1959 where Guihua oil field was found as a test oil field for the development of SPA oil field, Guihua has always been in operation for research and exploration and high production zones have been gradually found. The highest annual production of this oil field is 67,000 tons; accumulatively it has explored over 1.6 million tons of oil and the production of recent years is over 20,000 tons. It is found in comprehensive research of central Sichuan oil and gas resources that crossing and distorted belts of structure in regional inclines have good fractures and high production, which may boost the success of exploration.
Guihua oil field has a proven reserve and petroleum geological reserve of 67 million tons with the most proven reserve and production in central Sichuan oil fields. The exploration and development of this field have shown that good economic and social effects from complicated fractured reservoir may also be gained through untiring research, hard-working exploration, positive practice and continuous brainstorming. It will be proven by later new exploration of some old fields, including Longnvsi, Hechuan, Luodu, Guang’an, Xianduhe, Yingshan and Yilichang etc. (Fig 3)
Sichuan Basin, A Large Oildom in Southwest
Through half century of exploration in northeast of China, north China and northwest of China, Songliao, Bohai Bay, Ordos, Tarim, Junggar and Qaidamu large oildoms have been built with an annual oil production of 200,000,000 tons, but in south China there are only Jianghan and Jiangsu small oildoms with only over 2,000,000 tons of annual oil production. So, big safety problems have been caused by the transportation of crude oil and petroleum products from the north.
To speed up the oil exploration and production in Sichuan is very significant for the economic growth and petroleum supply in Sichuan, Chongqing and southwest, the relief of Oil North-South Transportation and the petroleum safety in China.
Then how to fulfill the target?
Firstly, we should emancipate our mind, update our conception, fully recognize the advantageous condition and resource potential for exploring oil in Sichuan recheck the petroleum resource and reserve of Sichuan basin, consider the potential of central Sichuan block and north, west, south and east Sichuan, confirm the resource of Da’anzhai, Lianggaoshan, Xujiahe and Shaximiao formations, and largely improve the oil resource in Sichuan basin.
Secondly, we should improve the precise prediction of carbonatite and sandstone reservoirs, consider the distribution rules of fractured reservoirs and the distribution characteristics of porous reservoirs, and confirm the distribution of river delta and subaqueous fan sandbody for the exploration of large structural lithologic hydrocarbon reservoirs.
Thirdly, we should figure out and make measures for keeping and increasing the production of low-permeability and low-porosity reservoirs. Most of the early high production wells are already in short-term off-production because the connectivity of underground reservoirs too poor to keep long-time high production. So, the start-up production of a well should be 1/3 or 1/4 of the test production, and great effort to keep production and fracture acidification for increasing production should be made according to the characteristics of reservoirs.
Fourthly, we should make wide open exploration, keep expanding the oil field and find out new fields to stably increase the oil reserve and production of Sichuan zone. Based on the situation that oil fields have been explored in anticline, syncline, incline and cap in central Sichuan block, we should research deep into the reservoir forming condition in different structures, especially into the possibility of multi-system overlapped hydrocarbon reservoirs in thrust faults.
Fifthly, we should devote to the scientific innovation and lead the oil exploration and development. The exploration of low-permeability and low-porosity reservoirs in Sichuan is a hard job, so we need to continuously improve our seismic, drilling and logging technologies, protect hydrocarbon layers and prevent pollution; we can drill more directional wells and horizontal wells and make the sand body penetrate longer and deeper in fracture sections to increase production. P+E