Origin and Distribution of Grain Dolostone Reservoirs in the Cambrian Longwangmiao Formation,Sichuan Basin,China

Dolostones in the Cambrian Longwangmiao Formation have become one of the most significant gas exploration domains in China. Over a trillion cubic meters of gas reservoirs have been discovered in the Gaoshiti-Moxi area; however, the origins and distribution of the dolostone reservoirs are not well understood. This work discussed the geology and geochemistry of the dolostone reservoirs in the Longwangmiao Formation to determine their origin and distribution. Two understandings are acquired： firstly, a carbonate ramp provided excellent conditions for grain beach deposition, while the presence of a hypersaline lake was favorable for the contemporaneous dolomitization of grain beach deposits. Petrographic and geochemical evidence further confirm that the Longwangmiao dolostone was formed during the contemporaneous stage. Secondly, the reservoir characteristics indicate that the grain beach sediments provide material basis for the development of the Longwangmiao dolostone reservoirs. Reservoir dissolution simulation experiments show that the porosity of the reservoirs was formed by dissolution during contemporaneous and burial stages. The dissolution pores formed during the contemporaneous stage were controlled by sequence interfaces. The large scale dissolution vugs formed during the burial stage subsequently spread along the pre-existing porosity and fracture zones. This study therefore identified that the development of grain dolostone reservoirs in a shallow water ramp under arid climatic conditions generally met the following conditions：（1） reefal beach deposits lay a foundation for reservoir development;（2） superficial conditions are an important determining factor for reservoir porosity; and（3） burial conditions provide environment for porosity preservation and modification.

Genesis of ultra-deep dolostone and controlling factors of largescale reservoir:A case study of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin

This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirs,focusing on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin.The study involves petrology,microscale X-ray diffraction,trace element analysis,and C-O-Sr-Mg isotope experiments to provide a detailed analysis.The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrix dolostone and four types of cement.The Dengying Formation,which developed under a sedimentary background of a restricted platform,contains special microbial and microcrystalline dolostones.The dolomite grains are small(<30μm)and have a low order degree(Min=0.55),with large unit cell parameters and an extremely high Na content(Max=788 ppm).The^(87)Sr/^(86)Sr value of the dolostone is consistent with contemporaneous seawater,while the δ^(13)C andδ^(18)O values are lower than those of the contemporaneous seawater.The δ^(26)Mg value is small(Min=-2.31‰).Powder crystal,fine-crystalline,and calcite dolostones with coarser and more ordered crystals exhibit similar δ^(13)C and^(87)Sr/^(86)Sr values to microbial and microcrystalline dolostone.During the sedimentary period of the Dengying Formation,ancient marine conditions were favorable for microbial survival.Microorganisms induced the direct precipitation of primary dolomite in seawater,forming microbial and microcrystalline dolostones during the seawater diagenesis period.During the subsequent diagenesis period,dolostones underwent the effects of dissolution-recrystallization,structures,and hydrothermal fluids.This resulted in the formation of dolostone with coarser crystals,a higher degree of order,and various types of cement.The Longwangmiao Formation was developed in an interplatform beach characterized by special particle dolostone.The particle dolostone has a large grain size(>30μm),high order degree(Min=0.7),small unit cell parameters,high Na content(Max=432 ppm),and low Fe and Mn content.The δ^(26)Mg and δ^(13)C values are consistent with the contemporaneous seawater,while the δ^(18)O and^(87)Sr/^(86)Sr values are higher than those of the contemporaneous seawater.There is mutual coupling between multiple-period varying δ^(26)Mg values and sedimentary cycles.The dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater.The thickness and scale of the dolostone in the Longwangmiao Formation are controlled by the periodic changes in sea level.The period of dolostone development from the Sinian to the Cambrian coincides with the transition from Rodinia’s breakup to Gondwana’s convergence.These events have resulted in vastly different marine properties,microbial activities,and sedimentary climate backgrounds between the Sinian and the Cambrian.These differences may be the fundamental factors leading to the distinct origins of dolostone formed in the two periods.The distribution of sedimentary facies and deep tectonic activities in the Sichuan Basin from the Sinian to the Cambrian is influenced by the breakup and convergence of the supercontinent.This process plays a key role in determining the distribution,pore formation,preservation,and adjustment mechanisms of ultra-deep dolostone reservoirs.To effectively analyze the genesis and reservoir mechanisms of ultra-deep dolostone in other regions or layers,especially during the specific period of supercontinent breakup and convergence,it is crucial to consider the comprehensive characteristics of seawater properties,microbial activities,sedimentary environment,and fault systems driven by tectonic activities.This can help predict the distribution of high-quality and large-scale ultra-deep dolostone reservoirs.

Dolomitization by Penesaline Sea Water in Early Cambrian Longwangmiao Formation, Central Sichuan Basin, China

The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum＇s precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitization may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high-and low-frequency sea-level changes. This kind of dolomitization can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity（normal, penesaline to hypersaline）.

Dolomite geochemistry of the Cambrian Longwangmiao Formation,eastern Sichuan Basin:Implication for dolomitization and reservoir prediction

Based on petrology and mineralogy,the geochemical characteristics of dolomite in the Cambrian Longwangmiao Formation in the eastern Sichuan Basin,were carried out.Results showed that dolomites mainly occurred in the middle and upper parts of Longwangmiao Formation,with most commonly as a crystalline dolomite and less commonly as a gain dolomite;the percentage content of dolostone in dolomite ranged from 48.6%to 75%,ratios of Mg/Ca were generally small,and positive and negative relationships between content of MgO and content of CaO were identified,respectively;the dolostones were generally characterized by high contents of Fe and Na while low contents of Sr and Mn,and only a part of dolostones had high content of Mn;compared with composition of contemporaneous seawater,most of dolostones were characterized by positiveδ^(13)C values and negativeδ^(18)O values.Based on the analyses of regional geological setting and geochemical characteristics,the dolomite in Longwangmiao Formation was mainly from the seepage-refluxion dolomitization,while evaporative-concentration dolomitization and burial dolomitization were not developed.In general,the dolomitization of Longwangmiao Formation was not complete.Furthermore,through the dolomitization model of Longwangmiao Formation,the distribution of sedimentary facies as well as the controlling effect of dolomitization on reservoir,the favorable reservoir was predicted,and was mainly developed around Qiyueshan fault zone.

Evaluation of dolomite reservoirs in the Longwangmiao Formation,Lower Cambrian in Northern Sichuan basin,China

With the extended petroleum exploration of the carbonate reservoir in the Longwangmiao Formation,Lower Cambrian in Sichuan basin,looking for a new favorable exploration area is the primary task in current exploration.This paper focusing on the dolomite reservoir in the Longwangmiao Formation in the Northern Sichuan basin,will evaluate the reservoir conditions,including reservoir lithology,reservoir spaces,physical properties and other basic characteristics.Combined with sedimentary environment study and diagenesis analysis,the genetic mechanism of Longwangmiao reservoir in study area will be discussed.It indicates that the reservoir rocks of Longwangmiao Formation in the study area are composed of dolarenite and crystalline dolomite,and the main reservoir spaces are intergranular dissolved pores,intragranular dissolved pores and intercrystalline dissolved pores.However,most of dissolved pores have been filled with bitumen,which lead to the reservoir properties very poor,both the porosity and permeability are worse than that of central Sichuan basin,so the reservoir in the Longwangmiao Formation in the Northern Sichuan basin is low-porosity and low-permeability reservoir.During the Early Cambrian Longwangmiao period,the study was near the old-land,and the energy of sea water was not enough to deposit many grain shoals,which cause the sediments are small with lots of quartz and muds.Moreover,because lacking of large scale karstification in the late diagenesis,there isn't much dissolved pores and vugs develop in the Longwangmiao Formation.Some Intergranular dissolved pores and intercrystalline dissolved pores formed by the early atmosphere water dissolved exist as some reservoir spaces,but most of them have been filled with bitumen,which was residual when the ancient oil reservoir transformed.Above factors cause the Longwangmiao Formation in the Northern Sichuan basin become not a very favorable reservoir in Sichuan basin.

Hydrocarbon origin and reservoir forming model research of Longwangmiao Formation,Moxi-Gaoshiti area,Sichuan Basin

This paper focuses on the Longwangmiao gas reservoir in Moxi-Gaoshiti area,Sichuan Basin.Starting from the tectonic evolution perspective,though comparing biological marker compound and analyzing fluid inclusions,the oil&gas origin and accumulation evolution of Longwangmiao Formation are systematic studied with reference to the burial-thermal evolution of single well geological history in the study area.It is suggested that the oil&gas reservoir is generally characterized by early accumulation,multi-stage filling,late cracking and later adjustment.The oil and gas were mainly sourced from lower Cambrian Qiongzhusi Formation,partly from the Permian source rock.During the geological period,3 major oil&gas fillings occurred in the Longwangmiao Formation,namely Caledonian-Hercynian filling that was small in scale and produced the first phase of paleo-oil reservoir that soon destroyed by Caledonian movement uplift,large-scale Permian filling that gave rise to the second-phase of paleo-oil reservoir and the Triassic-Jurassic filling that enriched the second phase of paleo-oil reservoir.Finally,the paleo-oil reservoir experienced an in-situ cracking during the cretaceous period that gave rise to a natural gas reservoir and left behind carbonaceous bitumen and oily bitumen in the holes of the Longwangmiao Formation.

Bitumen formation of Cambrian Longwangmiao Formation in the central Sichuan and its implication for hydrocarbon accumulation

Based on organic petrology,organic geochemistry and SEM method,type,formation period and source of bitumens in the Cambrian Longwangmiao Formation in the central Sichuan Basin were well investigated,and combined with fluid inclusions and tectonic evolution characteristics,the hydrocarbon accumulation history of the gas reservoir of the Longwangmiao Formation in the Anyue gasfield was also studied.The result shows that all bitumens in the Longwangmiao Formation was from the Lower Cambrian source rocks,it had multiple genetic types which was dominated by the pyrolysis genetic type;the bitumens were formed into three stages,i.e.,the bitumen of the oxidized water-washing type in the first stage,the bitumen of the precipitated type in the second stage,and the bitumen of the pyrolyzed type in the third stage;the gas reservoir in the Anyue gasfield experienced five stages of hydrocarbon charging,including two stages of liquid hydrocarbon charging,charging of the kerogen pyrolysis gas in the Late Triassic-Early Jurassic,charging of the crude oil pyrolysis gas in the Late Jurassic-Early Cretaceous and charging of the dry gas charging in the Himalayan trap reformation and adjustment process.

Gas accumulation conditions and key technologies for exploration & development of Sinian and Cambrian gas reservoirs in Anyue gasfield

Exploration of the Sinian-Lower Paleozoic formations in the Sichuan Basin starts from the 1950s,and is tortuous and complex.To summarize the exploration experiences is of great significance of reference for exploration of oil and gas in deep carbonate rocks of the Sichuan Basin.Gas accumulation in the Anyue oversize gasfield is chiefly controlled by the Deyang-Anyue intra-platform rift and central Sichuan paleouplift.The intra-platform rift controls the scale hydrocarbon generation center and effective hydrocarbon plays.The Anyue gasfield is characterized by near-source hydrocarbon accumulation.The marginal zone of intra-platform rift and high part of the paleouplift control the scale distribution of favorable facies belts of reservoir development,and overlapped by paleokarstic reformation,the reservoirs are widely distributed.The intra-platform rift and paleouplift jointly control formation of the scale traps and hydrocarbon enrichment.Gas reservoir features and major controlling factors for hydrocarbon accumulation have some differences in the main play strata,such as Longwangmiao Formation,and Member 4 and Member 2 of Dengying Formation.The gas reservoir in the Longwangmiao Formation is a lithologic gas reservoir under the tectonic setting,and the inherited paleouplift is a key for reservoir formation and hydrocarbon accumulation.The gas reservoir in the Member 4 of Dengying Formation is mainly controlled by large structural-stratigraphic composite traps,and the gas reservoir in the Member 2 of Dengying Formation is the structural trap.Distribution of the large gasfields in the Dengying Formation is controlled by the intra-platform rift,near-source hydrocarbon-reservoir configuration controlled by the ancient erosion plane and stratigraphic sealing in the updip direction.For exploration and development of the Anyue gasfield,four series of technologies are developed,namely,reservoir seismic fine description technology,well logging fine evaluation technology of gas layers,fast drilling technology of complex strata,and stimulation technology for high temperature and high pressure gas layers.Technical progresses provide powerful guarantee for efficient exploration and development of the oversize gasfields.