Geological Features and Reservoiring Mode of Shale Gas Reservoirs in Longmaxi Formation of the Jiaoshiba Area

This study is based on the sedimentation conditions, organic geochemistry, storage spaces, physical properties, lithology and gas content of the shale gas reservoirs in Longmaxi Formation of the Jiaoshiba area and the gas accumulation mode is summarized and then compared with that in northern America. The shale gas reservoirs in the Longmaxi Formation in Jiaoshiba have good geological conditions, great thickness of quality shales, high organic content, high gas content, good physical properties, suitable depth, good preservation conditions and good reservoir types. The quality shales at the bottom of the deep shelf are the main target interval for shale gas exploration and development. Shale gas in the Longmaxi Formation has undergone three main reservoiring stages：the early stage of hydrocarbon generation and compaction when shale gas reservoirs were first formed; the middle stage of deep burial and large-scale hydrocarbon generation, which caused the enrichment of reservoirs with shale gas; the late stage of uplift, erosion and fracture development when shale gas reservoirs were finally formed.

Tight sandstone gas accumulation mechanism and development models

Tight sandstone gas serves as an important unconventional hydrocarbon resource, and outstanding results have been obtained through its discovery both in China and abroad given its great resource potential. However, heated debates and gaps still remain regarding classification standards of tight sandstone gas, and critical controlling factors, accumulation mechanisms, and devel- opment modes of tight sandstone reservoirs are not deter- mined. Tight sandstone gas reservoirs in China are generally characterized by tight strata, widespread distri- bution areas, coal strata supplying gas, complex gas-water relations, and abnormally low gas reservoir pressure. Water and gas reversal patterns have been detected via glass tube and quartz sand modeling, and the presence of critical geological conditions without buoyancy-driven mecha- nisms can thus be assumed. According to the timing of gas charging and reservoir tightening phases, the following three tight sandstone gas reservoir types have been identified： （a） ＂accumulation-densification＂ （AD）, or the conventional tight type, （b） ＂densification-accumulation＂ （DA）, or the deep tight type, and （c） the composite tight type. For the AD type, gas charging occurs prior to reser- voir densification, accumulating in higher positions under buoyancy-controlled mechanisms with critical controlling factors such as source kitchens （S）, regional overlaying cap rocks （C）, gas reservoirs, （D） and low fluid potential areas （P）. For the DA type, reservoir densification prior to the gas charging period （GCP） leads to accumulation in depres- sions and slopes largely due to hydrocarbon expansive forces without buoyancy, and critical controlling factors are effective source rocks （S）, widely distributed reservoirs （D）, stable tectonic settings （W） and universal densification of reservoirs （L）. The composite type includes features of the AD type and DA type, and before and after reservoir densification period （RDP）, gas charging and accumulation is controlled by early buoyancy and later molecular expansive force respectively. It is widely distributed in anticlinal zones, deep sag areas and slopes, and is con- trolled by source kitchens （S）, reservoirs （D）, cap rocks （C）, stable tectonic settings （W）, low fluid potential areas （P）, and universal reservoir densification （L）. Tight gas resources with great resource potential are widely dis- tributed worldwide, and tight gas in China that presents advantageous reservoir-forming conditions is primarily found in the Ordos, Sichuan, Tarim, Junggar, and Turpan- Hami basins of central-western China. Tight gas has served as the primary impetus for global unconventional natural gas exploration and production under existing technical conditions.

Features of the Carboniferous Volcanic Rocks Fracture Reservoirs in Hongshanzui Oilfield,Junggar Basin

Based on comprehensive analysis of the drilling core, slice analysis, FMI image well log- ging and other well logging data, the features of the Carboniferous volcanic rocks fracture reservoirs in Hongshanzui oilfield of Junggar Basin have been studied. According to contributing factor, the fracture reservoirs were divided into diagenetic fractures, tectonic fractures, weathered fractures and dissolving fractures. And the diagenetic fractures were further divided into three subclasses of conden- sation contraction fractures, intergravel fractures and intercrystal fractures. The fracture reservoirs were also divided into 4 categories, such as high-angle fractures, oblique fractures, mesh fractures and low-angle fractures according to fracture attitude. With the FMI image logging and other Jogging in- formation, the scale logging by the core method was adopted, our study shows that the fracture of tuff was the most developed, and the fracture density can be reached 11.46 bar/m, followed by the lava, and the sedimentary rock fracture was the lowest level. The fractures thickness in the region of 300 m apart from weather crust was accounted for 80.3% of the total thickness. Because of image logging data is li- mited, the method of drilling wells trajectory to calculate the fracture trend was proposed. The fracture trend was divided into two groups NE and NW. The secondary pore-fracture forming and distribution were controlled by fracture system, formed the beneficial passage system of the oil and gas, the fracture system was the key factor in restricting oil and gas accumulation, and was crucial for the volcanic rocks reservoirs.

Bioherm Petroleum Reservoir Types and Features in Main Sedimentary Basins of the South China Sea

It is known from macrocomparisons and microresearches of bioherm reservoirs in main sedimentary basins of the South China Sea through deep-water petroleum explorations and by means of 2D/3D seismic data and a whole-coring core from the Xisha Islands that there are great dif- ferences between deep-sea oil and gas fields in the world and those in the South China Sea, as reservoir systems of the former are mainly clastic rocks, whereas the latter have organic reefs that act as reser- voirs of their largest oil and gas fields, which are represented by large Liuhua 11-1 reef oilfield in the north and super-large L reef gas field in the south of the South China Sea. Therefore, it is of great significance to study deep-water hioherm reservoirs in the South China Sea. Comparisons of organic reefs in the four large islands of the South China Sea give evidences that such reefs in main sedimentary basins came into being during Cenozoic, especially in Neogene, and mainly occur as tower （point） reef, massive reef, platform-edge reef, and patch reef in shape, which show different reservoir physical properties and seismic reflection configurations and make up carbonate rock-bioherm formations in the island reef and sedimentary basin areas. Generally, the south and north parts differ from the east and the west of the South China Sea in geologic conditions, as their corresponding continental shelf/island shelf areas are relatively wide/ narrow, large stream current systems are well developed/not so well developed, and terrigenous sediments are relatively sufficient/insufficient. The southeast and south parts of the South China Sea had organic reefs built up earlier than the north and the reef building mainly took place in Neogene; these Neogene organic reefs all be- long to plant algal reef rocks. Liuhua oilfield in the Pearl River Mouth basin is found to mainly have red algal bindstone, Malampaya reef in the northern Palawan basin is rich in both red algal bind- stone and green algal reef segmented rock, and especially Miocene red algal framestone and green algal segmented rock are discovered in the Xisha Islands. These algal reefs created different sedimentary mi- crofacies as well as various rock structures and types, and through recent researches on the mechanism of dolomitization, freshwater dolomite was discovered and grouped under products from dolomitization in mixed water that was regression reefal dolomite of good reservoir properties.

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.