In recent years, gas reservoirs containing high levels of hydrogen sulfide have been found in salinized lacustrine carbonate rocks in the upper member of the Lower Ganchaigou Formation (E 3 2 ) in the Yingxiongling area of the western Qaidam Basin. This poses great safety risks to drilling and development. To clarify the genetic mechanisms of hydrogen sulfide, we have analyzed the geochemical characteristics and sulfur isotopes of natural gas and predicted the distribution of hydrogen sulfide. The results show that (1) natural gas in the Yingxiongling area is dominated by wet gas, with relatively high nonhydrocarbon content. The gas is E 3 2 oil-type gas in a mature stage. (2) Hydrogen sulfide in natural gas reservoirs is less likely to originate from biological and volcanic geneses, and it is primarily a product of thermochemical sulfate reduction. The high-quality Oligocene source rocks distributed across a large area, the saline-deposit formations widely distributed in the western Qaidam Basin, and the high geothermal gradients and large numbers of developed pore-type reservoirs, all combine to provide favorable conditions for the formation of hydrogen sulfide reservoirs. (3) During the E 3 2 deposition period, rock salt developed in the center of the lake basin, the sealing conditions for oil and gas were favorable, and the formation temperature was high during the reservoir formation period. Development of subsalt porous reservoirs is conducive to enrichment of hydrogen sulfide, and comprehensive prediction indicates that the subsalt formation in the Central Yingxiongling-Ganchaigou area is an enrichment zone for hydrogen sulfide. The research results are of considerable significance for the exploration and deployment, and the production safety, of hydrogen sulfide-containing reservoirs.
Abstract. The Lower Cretaceous Bashijiqike Formation of the Kuqa Depression is made up of ultra-deeply buried sandstones in fold-and-thrust belts. Few researches have linked diagenetic processes with structure. To fill this gap, a comprehensive analysis integrating diagenesis with structure pattern, fracture and in situ stress is performed following a structural diagenetic approach. The results show that the pore spaces include residual intergranular pores, intergranular and intragranular dissolution pores, and micro-fractures. The sandstones experienced a high degree of mechanical compaction, but compaction is limited in well-sorted rocks or abundant in rigid quartz grains. The most volumetrically important diagenetic minerals are calcites. The framework grains experienced a varied degree of dissolution, and intergranular and intragranular dissolution pores are formed. Special attention is paid on the dissolution associated with the fracture planes. Large numbers of natural fractures are cemented by carbonate cements, which limit fluid flow. In addition, the presence of fracture enhances dissolution and the fracture planes are enlarged by dissolution. Cementation and dissolution can occur simultaneously in fracture surfaces, and the enlarged fracture surfaces can be cemented by late-stage cements. The in situ stress magnitudes are calculated using well logs. The horizontal stress difference (Δσ) determines the degree of mechanical compaction, and rocks associated with low Δσ experienced a low degree of compaction, and these contain preserved intergranular pores. Natural fractures are mainly related to the low Δσ layers. The presence of intergranular and intragranular dissolution pores is mainly associated with the fractured zones. The high-quality reservoirs with intergranular pores or fractures are related to low Δσ layers. The structural diagenesis researches above help the prediction of reservoir quality in ultra-deep sandstones and reduce the uncertainty in deep natural gas exploration in the Kuqa Depression.
In our country as the lower frequency of land and shallow water exploration for oil and gas,deep water oil and exploration is playing more important role.Big reservoir body becomes the target of deepwater oil and exploration,because the deepwater oil and exploration has some characteristics,such as greater risks and higher investment.Central canyon,which comes from Yinggehai basin and pass through Qiongdongnan basin and enter into the xisha trough,lies in the deepwater area of Qiongdongnan basin and is mainly formed at 10.5 Ma,5.5 Ma and 4.2 Ma.With an area of more than 50 000 km2,central canyon can be a target of deepwater oil and exploration. In the central canyon,levee-overbank sediments are abundant.Deepwater levee-overbank has received considerable attention in the petroleum industry because of having good reservoirs.In order to conduct the prospect for central canyon,this article has analyzed the shape and control factors of levee-overbank deposited in the central canyon by using 3D seismic profile and RMS attribute.After that,the sediment model is summed up and the foreground of prospect is analyzed.The result has shown below:1.levee-overbank sediments in the central canyon developed at least eight times,and have many micro facies such as crevasse splay,overflow splay and levee.In the cross seismic profiles,levee-overbank sediments represent wedge shape with strong amplitude,intermediate frequency and mid-good continuity,and combining with channel has shown gull-wing shape.In the plane,the overall shape of levee-overbank sediments is elongate and trends roughly parallel to the channel.Its area of single sand body could reach 17 km2. 2.In the central canyon,levee-overbank sediments is mainly controlled by province and ancient physiognomy.The grain size whether sand-rich or mud-rich is mainly decided by province.The position is mainly decided by ancient physiognomy.The pond accommodate space is afforded for levee-overbank to be deposited in the area where has fluctuation variation.On the opposite,in the incline area,there is no accommodation and levee-overbank cannot deposit. 3.Levee-overbank sediments which has many deposition time and large single sand body can be ideal stratigraphic traps and it could accumulate oil and if it cooperate with fault or gas chimney.Levee-overbank sediments have well foreground for prospect.
Some facies marks of submerged sedimentary (such as shale color and palaeobiological fossil) and reworking marks of lake current (such as lithology and texture, sedimentary structure, grain size analysis, litholic composition and its maturity, facies sequence in the cores, plane pattern of sand body) have been found in the lower Guantao Formation of the Neogene in the eastern Jiyang Depression. It is proved that Guantao Formation of the Neogene in Jiyang Depression is a braided river delta sedimentation rather than fluvial facies. The braided river delta has developed submerged braided river, river mouth bar, frontal sheet sand and other microfacies. This new discovery is of great significance for oil exploration and production of the Neogene in Jiyang Depression.
Lower Silurian Longmaxi formation (LSL) shale is widely and continuously distributed in the northeastern Sichuan Basin and, based on structural analogies with the gas producing LSL formation in the southeastern Sichuan Basin, has significant potential for shale gas exploration. However, limited research has been performed to evaluate the shale gas potential in this region. Samples from a recently completed exploratory well (Well-WQ2) in the northeastern Sichuan Basin indicate that the LSL shale has a vertical property sequence that closely resembles the vertical property sequences in wells in the gas-producing sections of the southeastern Sichuan Basin. The continuous sampling and analyses of Well-WQ2 have allowed a detailed investigation of the vertical variations in lithofacies, mineral characteristics, pore structures, and organic geochemical characteristics. The Longmaxi formation was divided into two third-order sequences (SQ1 and SQ2) based on systematic core observations and well logging analyses. Both SQ1 and SQ2 include a transgressive system tract (TST) and a high-stand system tract (HST). The lithofacies exhibit an upward decrease in the organic content. From SQ1 to SQ2, the quartz content, in situ graptolite content, total organic carbon (TOC) content, and brittleness index decrease, but the clay mineral content increases. The LSL shale sections from depths of 1204 to 1214 m and from 1271 to 1282 m possess well-developed fractures and high permeability. Additionally, the average porosity and permeability in SQ1 are higher than those in SQ2. In addition, the positive correlation between the TOC and quartz contents of the assayed samples suggests that much of the quartz is of biogenic origin. Changes in the sedimentary and diagenetic environments during deposition are two key factors that contribute to the observed vertical heterogeneity of the Longmaxi formation. In conclusion, the shale sections of the lower part of the SQ1, like their analogs in the southeastern Sichuan Basin, are the most favorable targets for shale gas production in the northeastern Sichuan Basin.
Summary Petrophysical properties are important characteristic parameters of reservoir quality. Rock physics models provide the link between elastic parameters and reservoir properties of interest. Pre-stack seismic inversion is an efficient approach to obtain these petrophysical parameters. This study proposes a direct estimation method of petrophysical properties based on AVO inversion. Firstly, a linear regression analysis of the well data is conducted and the rock physics model is obtained. Secondly, we derived the reflection coefficient approximate equation in terms of porosity, shale content and density through the incorporation of the rock physics model into the linear expression of Aki's reflection coefficient. The equation establishes the theory foundation for the inversion of reservoir properties. Finally, we introduce a robust AVO inversion procedure to invert for reservoir parameters directly. The AVO inversion of the model and real data shows that the proposed inversion method can obtain reliable reservoir parameters from the seismic data directly.
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