High-resolution seismic characterization of the gas and gas hydrate system at Green Canyon 955, Gulf of Mexico,USA

The Pliocene and Pleistocene sediments at lease block Green Canyon 955 (GC955) in the Gulf of Mexico include sand-rich strata with high saturations of gas hydrate; these gas hydrate accumulations and the associated geology have been characterized over the past decade using conventional industry three-dimensional (3D) seismic data and dedicated logging-while-drilling (LWD) borehole data. To improve structural and stratigraphic characterization and to address questions of gas flow and reservoir properties, in 2013 the U.S. Geological Survey acquired high-resolution two-dimensional (2D) seismic data at GC955. Combined analysis of all available data improves our understanding of the geological evolution of the study area, which includes basin-scale migration of the Mississippi River sediment influx as well as local-scale shifting of sedimentary channels at GC955 in response to salt-driven uplift, structural deformation associated with the salt uplift, and upward gas migration from deeper sediments that charges the main gas hydrate reservoir and shallower strata. The 2D data confirm that the sand-rich reservoir is composed principally of sediments deposited in a proximal levee setting and that episodes of channel scour, interspersed with levee deposition, have resulted in an assemblage of many individual proximal levee deposit “pods” each with horizontal extent up to several hundred meters. Joint analysis of the 2D and 3D data reveals new detail of a complex fault network that controls the fluid-flow system; large east-west trending normal faults allow fluid flow through the reservoir-sealing fine-grained unit, and smaller north-south oriented faults provide focused fluid-flow pathways (chimneys) through the shallower sediments. This system has enabled the flow of gas from the main reservoir to the seafloor throughout the recent history at GC955, and its intricacies help explain the distributed occurrences of gas hydrate in the intervening strata.     

轮南地区压力场演化及其成藏意义

选择轮南主体区剖面七条和临近凹陷剖面两条，利用压力数值模拟手段，对轮南地区的压力场演化进行了恢复和模拟。结合实测压力资料和钻井资料，发现部分地区石炭系和上奥陶统中存在弱超压、寒武系在凹陷内存在超压。通过分析，石炭系超压在轮南主体区主要是水热增压作用和泥岩转换作用造成的，在凹陷内则可能主要由于欠压实引起。上奥陶统内部的超压和石炭系在形成时间、特征和发育机理上相似，不过轮南地区上奥陶统覆盖区域较少。寒武系超压和三个成藏期相对应，在凹陷内形成明显的三期超压现象，以喜山期超压幅度最大。整体上看，石炭系和上奥陶统弱超压，对成藏动力方面贡献不大，但是超压的形成增强了该层的封闭性，对喜山期天然气成藏有重要影响。寒武系超压为油气初次运移提供了动力，但是超压幅度有限，推测其在油气二次运移过程中贡献不大。     

Impact of sedimentology,diagenesis, and solid bitumen on the development of a tight gas grainstone reservoir in the Feixianguan Formation,Jiannan area,China: Implications for gas exploration in tight carbonate reservoirs

Tight gas grainstone reservoirs in the third member of the Feixianguan Formation, Jiannan area, evolved from a paleo-oil accumulation as evidenced from abundant solid reservoir bitumen. Porosity evolution of the grainstones was studied by evaluating relative influences of sedimentology, diagenesis, and solid bitumen formed during cracking of accumulated oils. Grainstones exhibited regional-distinct effectiveness for paleo-oil and present-gas accumulations during oil window and subsequent gas window diagenesis. In the southern zone where grainstones were not subjected to subaerial exposure and meteoric diagenesis in the early diagenetic stage, paleoporosity at the time of oil charge was mainly controlled by sedimentologic factors (e.g., grain size, sorting, and grain type), and paleo-oil reservoirs only occurred in the ooid-dominated grainstones with good sorting and coarse grain size. In contrast, in the northern zone meteoric diagenesis was responsible for paleoporosity preservation due to the early mineral stabilization of grains and meteoric calcite cementation, which caused grainstones greater resistance to compaction. Hence, most of the grainstones in the northern zone, regardless of textural variables, formed effective reservoirs for paleo-oil accumulation. As the oil cracked to gas with increasing depth and temperature during the late oil window and initial gas window, solid bitumen occluded reservoir pores to varying degrees and caused paleo-oil reservoirs to be significantly heterogeneous or completely ineffective for gas accumulation. In contrast, most grainstones that were once ineffective oil reservoirs transformed into effective gas reservoirs due to no or minor influence of solid bitumen precipitation. The model of reservoir transformation development of tight grainstones provides a plausible explanation for key observations concerning the diagenetic and distribution differences between paleo-oil and present-gas reservoirs. It is useful in predicting the distribution of potential reservoirs in carbonate strata in future exploration.     

Reservoir characterization of gas hydrate in the Northwestern part of the Ulleung Basin,East Sea

An analysis of 3D seismic data from the northwestern part of the Ulleung Basin, East Sea, revealed that the gas hydrate stability zone (GHSZ) consists of five seismic units separated by regional reflectors. An anticline is present that documents activity of many faults. The seismic indicators of gas hydrate occurrence included bottom simulating reflector (BSR) and acoustic blanking in the gas hydrate occurrence zone (GHOZ). By the analysis of the seismic characteristics and the gradient of the sedimentary strata, the GHOZ was divided into four classes: (1) dipping strata upon strong BSR, (2) dipping strata below strong BSR, (3) parallel strata with acoustic blanking, and (4) parallel strata below weak BSR. Seismic attributes such as reflection strength and instantaneous frequency were computed along the GHOZ. Low reflection strength and high instantaneous frequency were identified above the BSR, indicating the occurrence of gas hydrate. A remarkably high reflection strength and low instantaneous frequency indicated the presence of free gas below the BSR. Considering the distribution of the gas hydrate and free gas, two gas migration processes are suggested: (1) stratigraphic migration through the dipping, permeable strata and (2) structural migration from below the GHSZ along faults.     

High-resolution seismic imaging of gas accumulations and seepage in the sediments of the Ria de Aveiro barrier lagoon (Portugal)

Methane is a powerful greenhouse gas and an important energy source. The global significance and impact in coastal zones of methane gas accumulation and seepage in sediments from coastal lagoon environments are still largely unknown. This paper presents results from four high-resolution seismic surveys carried out in the Ria de Aveiro barrier lagoon (Portugal) in 1999, 2002 and 2003. These comprise three chirp surveys (RIAV99, RIAV02, RIAV02A) and one boomer survey (RIAV03). Evidence of extensive gas accumulation and seepage in tidal channel sediments from the Ria de Aveiro barrier lagoon is presented here for the first time. This evidence includes: acoustic turbidity, enhanced reflections, acoustic blanking, domes, and acoustic plumes in the water column (flares). The stratigraphy and structural framework control the distribution and extent of gas accumulations and seepage in the study area. In these shallow systems, however, tidal altitude variations have a significant impact on gas detection using acoustic methods, by changing the raw amplitude of the enhanced seismic reflections, acoustic turbidity, and acoustic blanking in gas-prone areas. Direct evidence of gas escape from drill holes in the surrounding area has shown that the gas present in the Ria de Aveiro consists of biogenic methane. Most of the gas in the study area was probably generated mainly in Holocene lagoon sediments. Evidence of faults affecting the Mesozoic limestones and clays underlying some of the shallow gas occurrences, and the presence of high-amplitude reflections in these deeper units raise the possibility that some of this gas could have been generated in deeper sedimentary layers, and then migrated upward through the fractured Mesozoic strata.     

Controlling factors on the charging process of oil and gas in the eastern main sub-sag of the Baiyun Sag,Zhujiang River (Pearl River) Mouth Basin

The eastern main sub-sag (E-MSS) of the Baiyun Sag was the main zone for gas exploration in the deep-water area of the Zhujiang River (Pearl River) Mouth Basin at its early exploration stage, but the main goal of searching gas in this area was broken through by the successful exploration of the W3-2 and H34B volatile oil reservoirs, which provides a new insight for exploration of the Paleogene oil reservoirs in the E-MSS. Nevertheless, it is not clear on the distribution of “gas accumulated in the upper layer, oil accumulated in the lower layer” (Gas_upper-Oil_lower) under the high heat flow, different source-rock beds, multi-stages of oil and gas charge, and multi-fluid phases, and not yet a definite understanding of the genetic relationship and formation mechanism among volatile oil, light oil and condensate gas reservoirs, and the migration and sequential charge model of oil and gas. These puzzles directly lead to the lack of a clear direction for oil exploration and drilling zone in this area. In this work, the PVT fluid phase, the origin of crude oil and condensate, the secondary alteration of oil and gas reservoirs, the evolution sequence of oil and gas formation, the phase state of oil and gas migration, and the configuration of fault activity were analyzed, which established the migration and accumulation model of Gas_upper-Oil_lower co-controlled by source and heat, and fractionation controlled by facies in the E-MSS. Meanwhile, the fractionation evolution model among common black reservoirs, volatile reservoirs, condensate reservoirs and gas reservoirs is discussed, which proposed that the distribution pattern of Gas_upper-Oil_lower in the E-MSS is controlled by the generation attribute of oil and gas from source rocks, the difference of thermal evolution, and the fractionation controlled by phases after mixing the oil and gas. Overall, we suggest that residual oil reservoirs should be found in the lower strata of the discovered gas reservoirs in the oil-source fault and diapir-developed areas, while volatile oil reservoirs should be found in the deeper strata near the sag with no oil-source fault area.     

南海神狐海域天然气水合物饱和度的数值模拟分析

珠江口盆地神狐海域是天然气水合物钻探和试验开采的重点区域,大量钻探取心、测井与地震等综合分析表明不同站位水合物的饱和度、厚度与气源条件存在差异。本文利用天然气水合物调查及深水油气勘探所采集的测井和地震资料建立地质模型,利用PetroMod软件模拟地层的温度场、有机质成熟度、烃源岩生烃量、流体运移路径以及不同烃源岩影响下的水合物饱和度,结果表明:生物成因气分布在海底以下1500 m范围内的有机质未成熟地层,而热成因气分布在深度超过2300 m的成熟、过成熟地层。水合物稳定带内生烃量难以形成水合物,形成水合物气源主要来自于稳定带下方向上运移的生物与热成因气。模拟结果与测井结果对比分析表明,稳定带下部生物成因气能形成的水合物饱和度约为10%,在峡谷脊部的局部区域饱和度较高;相对高饱和度(>40%)水合物形成与文昌组、恩平组的热成因气沿断裂、气烟囱等流体运移通道幕式释放密切相关,W19井形成较高饱和度水合物的甲烷气体中热成因气占比达80%,W17井热成因气占比为73%,而SH2井主要以生物成因为主,因此,不同站位甲烷气体来源占比不同。     

Stable carbon isotope ratios of CH4-rich gas inclusions in shale-hosted fracture-fill mineralization: A tool for tracing hydrocarbon generation and migration in shale plays for oil and gas

Fluid inclusion gases in minerals from shale hosted fracture-fill mineralization have been analyzed for stable carbon isotopic ratios of CH₄ using a crushing device interfaced to an isotope ratio mass spectrometer (IRMS). The samples of Paleozoic strata under study originate from outcrops and wells in the Rhenish Massif and Campine Basin, Harz Mountains, and the upper slope of the Southern Permian Basin. Fracture-fill mineralization hosted by Mesozoic strata was sampled from drill cores in the Lower Saxony Basin. Some studied sites are candidates for shale gas exploration in Germany. Samples of Mesozoic strata are characterized by abundant calcite-filled horizontal fractures which preferentially occur in TOC-rich sections of the drilled sediments. Only rarely are vertical fractures filled with carbonates and/or quartz in drill cores from Mesozoic strata but in Paleozoic shale they occur frequently. The δ¹³C(CH₄) values of fluid inclusions in calcite from horizontal fractures hosted by Mesozoic strata suggest that gaseous hydrocarbons were generated during the oil/early gas window and that the formation of horizontal fractures seems to be related to hydraulic expulsion fracturing. The calculated maturity of the source rocks at the time of gas generation lies below the maturity derived from measured vitrinite reflectance. Thus, the formation of horizontal fractures and trapping of gas that was generated in the oil and/or early gas window obviously occurred prior to maximal burial. Rapidly increasing vitrinite reflectance data seen locally can be explained by hydrothermal alteration, as indicated by increasing δ¹³C (CH₄–CO₂) values in fluid inclusions. The formation of vertical fractures in studied Mesozoic sediments is related to stages of post-burial inversion; gas-rich inclusions in fracture filling minerals recorded the migration of gas that had probably been generated instantaneously, rather than cumulatively, from high to overmature source rocks. Since no evidence is given for the presence of early generated gas in studied Paleozoic shale, it appears likely that major gas loss from shales occurred due to deformation and uplift of these sediments in response to the Variscan Orogeny.     

Estimation of seismic velocities and gas hydrate concentrations: A case study from the Shenhu area,northern South China Sea

In the Shenhu area of the northern South China Sea (SCS), canyon systems and focused fluid flow systems increase the complexity of the gas hydrate distribution in the region. It also induces difficulties in predicting the hydrate reservoir characteristics and quantitatively evaluating reservoir parameters. In this study, several inversion methods have been executed to estimate the velocities of strata and gas hydrate concentrations along a profile in the Shenhu area. The seismic data were inverted to obtain the reflection coefficient of each stratum via a spectral inversion method. Stratigraphic horizons were then delineated by tracking the inverted reflectivities. Based on the results of spectral inversion, a low-frequency velocity field of the strata was constructed for acoustic impedance inversion. Using a new iterative algorithm for acoustic impedance inversion, reflection coefficients were converted into velocities, and the velocity variations of the strata along a 2D seismic line were then obtained. Subsequently, gas hydrate saturations at well SH2 were estimated via the shale-corrected resistivity method, the chloride ion concentration method and three different rock physics models. The results were then compared to determine the optimal rock physics model, and the modified Wood equation (MWE) was found to be appropriate for this area. Finally, the inverted velocities and MWE were used to predict the distribution and concentrations of gas hydrates along the seismic line. The estimated spatial distribution of gas hydrates is consistent with that from sonic logging and resistivity data at well SH2, and with the drilling results. Therefore, this method is applicable in areas with no well data, or with few wells, and provides an effective tool for predicting and evaluating gas hydrates using seismic data.     

Seismic expression of gas and gas hydrates across the western Black Sea

This study is a synthesis of gas-related features in recent sediments across the western Black Sea basin. The investigation is based on an extensive seismic dataset, and integrates published information from previous local studies. Our data reveal widespread occurrences of seismic facies indicating free gas in sediments and gas escape in the water column. The presence of gas hydrates is inferred from bottom-simulating reflections (BSRs). The distribution of the gas facies shows (1) major gas accumulations close to the seafloor in the coastal area and along the shelfbreak, (2) ubiquitous gas migration from the deeper subsurface on the shelf and (3) gas hydrate occurrences on the lower slope (below 750 m water depth). The coastal and shelfbreak shallow gas areas correspond to the highstand and lowstand depocentres, respectively. Gas in these areas most likely results from in situ degradation of biogenic methane, probably with a contribution of deep gas in the shelfbreak accumulation. On the western shelf, vertical gas migration appears to originate from a source of Eocene age or older and, in some cases, it is clearly related to known deep oil and gas fields. Gas release at the seafloor is abundant at water depths shallower than 725 m, which corresponds to the minimum theoretical depth for methane hydrate stability, but occurs only exceptionally at water depths where hydrates can form. As such, gas entering the hydrate stability field appears to form hydrates, acting as a buffer for gas migration towards the seafloor and subsequent escape.     

台西盆地致密砂岩气存在的可能性

以致密砂岩气藏成藏机理为指导,综合利用烃源岩有机地化和储层孔渗参数等资料,对台西盆地致密砂岩气藏发育条件进行了探索性研究。在概括烃源岩、储集层及封盖保存等基本地质条件对油气聚集控制的基础上,通过分析致密砂岩气藏的形成机理,认为台西盆地具备良好的致密砂岩气藏发育条件,存在巨大的天然气勘探潜力,盆地东部陆区白垩系—中新统下部、西部海域南日坳陷白垩系—始新统、澎湖坳陷始新统下部为致密砂岩气藏发育有利层位,是台西盆地天然气勘探的新领域。     

The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain,Northwest China

Muri Basin in the Qilian Mountain is the only permafrost area in China where gas hydrate samples have been obtained through scientific drilling. Fracture-filling hydrate is the main type of gas hydrate found in the Qilian Mountain permafrost. Most of gas hydrate samples had been found in a thin-layer-like, flake and block group in a fracture of Jurassic mudstone and oil shale, although some pore-filling hydrate was found in porous sandstone. The mechanism for gas hydrate formation in the Qilian Mountain permafrost is as follows: gas generation from source rock was controlled by tectonic subsidence and uplift--gas migration and accumulation was controlled by fault and tight formation--gas hydrate formation and accumulation was controlled by permafrost. Some control factors for gas hydrate formation in the Qilian Mountain permafrost were analyzed and validated through numerical analysis and laboratory experiments. CSMGem was used to estimate the gas hydrate stability zone in the Qilian permafrost at a depth of 100–400 m. This method was used to analyze the gas composition of gas hydrate to determine the gas composition before gas hydrate formation. When the overlying formation of gas accumulation zone had a permeability of 0.05 × 10⁻¹⁵ m² and water saturation of more than 0.8, gas from deep source rocks was sealed up to form the gas accumulation zone. Fracture-filling hydrate was formed in the overlap area of gas hydrate stability zone and gas accumulation zone. The experimental results showed that the lithology of reservoir played a key role in controlling the occurrence and distribution of gas hydrate in the Qilian Mountain permafrost.     

Impacts of the tectonic stress field on natural gas migration and accumulation: A case study of the Kuqa Depression in the Tarim Basin,China

Stress, fluid and temperature are three of the major factors that impact natural gas migration and accumulation. In order to study the influences of tectonic stress field on natural gas migration and accumulation in low-permeability rocks, we take the Kuqa Depression as an example and analyze the evolution of the structure and tectonic stress field at first. Then we study the influences of tectonic stress field at different tectonic episodes on fractures and fluid potentials through the numerical simulation method on the section across the KL2 gas field. We summarize two aspects of the impact of the tectonic stress field on natural gas migration and accumulation. Firstly, under the effects of the tectonic stress field, the rock dilation increases with the added stress and strain, and when the shear stress of rock exceeds its shear strength, the shear fractures are well developed. On one hand, the faults which communicate with the hydrocarbon source rocks become the main pathways for natural gas migration. On the other hand, these positions where fractures are well developed near faults can become good reservoirs for natural gas accumulation. Secondly, because fluid potentials decrease in these places near the faults where fractures are well developed, natural gas can migrate rapidly along the faults and accumulates. The impact of tectonic stress fields on natural gas migration and accumulation allows for hydrocarbon migration and accumulation in the low-permeability rocks in an active tectonic compressive setting.     

Modeling of rising methane bubbles during production leaks from the gas hydrate sites of India

Natural gas hydrates is considered as a strategic unconventional clean hydrocarbon resource in the energy sector. Understanding the behavior of the rising methane gas bubbles during production leaks from the deep marine gas hydrate reservoirs well head is essential for environmental impact studies and to design environmental monitoring systems. Numerical model for quantitatively characterizing the vertical dissolution pattern of the wellhead released methane gas bubbles is analyzed for three potential gas hydrate locations in India. Simulation results indicate that the methane bubbles with diameter of 10?mm can transport methane gas till 650, 800, and 750?m from the seabed in the Krishna–Godavari(KG), Mahanadi and Andaman basins respectively. Results brought out that potential well head damage during methane hydrate production at 1050?m water depth could release up to 28?m³ of methane gas, in which 50% of the molar mass shall get dissolved within 40?m of water column from the seafloor.     

Spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the Yinggehai Basin,offshore South China Sea

About 120 gas seepage vents were documented along the west and southwest coast of the Hainan Island, South China Sea, in water depths usually less than 50 m. The principal seepage areas include the Lingtou Promontory, the Yinggehai Rivulet Mouth, Yazhou Bay, the Nanshan Promontory and the Tianya Promontory. They occur along three major zones, reflecting the control by faults and lateral conduits within the basement. It is estimated that the total gas emission from these seepage vents is 294–956 m³/year. The seepage gases are characterized by a high CH₄ content (76%), heavy δ¹³C₁ values (−38 to −33‰) and high C₁/C_1–5 ratios (0.95–1.0), resembling the thermogenic gases from the diapiric gas fields of the Yinggehai Basin. Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression. The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages. It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin. This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.     

南黄海盆地中、古生界油气地质条件研究

南黄海盆地在以中、新生界为主开展的油气勘探历时近30年,至今未获工业油气流。而盆地中、古生界石油勘探程度很低．目前仅只有少数钻井钻遇中、古生界,对其基本油气地质条件认识存在不足。此文运用含油气系统理论和盆地模拟技术,利用钻井资料,类比下扬子陆区石油地质条件研究成果,对下扬子南黄海盆地中、古生界海相地层烃源岩、储层和盖层条件进行了初步研究,为该盆地中、古生界下一步油气勘探指出了有利方向。     

Source and accumulation of gas hydrate in the northern margin of the South China Sea

The northern South China Sea (NSCS) experienced continuous evolution from an active continental margin in the late Mesozoic to a stable passive continental margin in the Cenozoic. It is generally believed that the basins in the NSCS evolved as a result of Paleocene–Oligocene crustal extension and associated rifting processes. This type of sedimentary environment provides a highly favourable prerequisite for formation of large-scale oil- and gas–fields as well as gas hydrate accumulation. Based on numerous collected data, combined with the tectonic and sedimentary evolution, a preliminary summary is that primitive coal-derived gas and reworked deep gas provided an ample gas source for thermogenic gas hydrate, but the gas source in the superficial layers is derived from humic genesis. In recent years, the exploration and development of the NSCS oil, gas and gas hydrate region has provided a basis for further study. A number of 2D and 3D seismic profiles, the synthetic comparison among bottom simulating reflector (BSR) coverage characteristics, the oil-gas area, the gas maturity and the favourable hydrate-related active structural zones have provided opportunities to study more closely the accumulation and distribution of gas hydrate. The BSR has a high amplitude, with high amplitude reflections below it, which is associated with gas chimneys and pockmarks. The high amplitude reflections immediately beneath the BSR are interpreted to indicate the presence of free gas and gas hydrate. The geological and geochemical data reveal that the Cenozoic northern margin of the NSCS has developed coal-derived gas which forms an abundant supply of thermogenic gas hydrate. Deep-seated faults and active tectonic structures facilitate the gas migration and release. The thermogenic gas hydrate and biogenic gas are located at different depths, have a different gas source genesis and should be separately exploited. Based on the proven gas hydrate distribution zone, we have encircled and predicted the potential hydrate zones. Finally, we propose a simple model for the gas hydrate accumulation system in the NSCS Basin.     

The biogenic gas potential of the submarine canyon systems of Plio-Peistocene foreland Basin,southwestern Taiwan

Biogenic gas was accidentally discovered and produced from the Plio/Pleistocene formation of the Hsinying gas field in southwestern Taiwan in 1989. A stratigraphic trapping mechanism related to the evolution of submarine canyon systems in the Plio-Peistocene foreland basin has been proposed in a previous study which explained underestimated recoverable gas reserve before drilling. To verify this shallow gas exploration hypothesis and to systematically evaluate the biogenic gas generation and entrapment potential of the submarine canyon systems, seismic interpretation, high-resolution sequence stratigraphic interpretation, seismic attribute analysis and geochemical analysis were performed and integrated in this study. Twenty-nine submarine canyons mapped mainly trend in a NE direction, except the NW trending Eurchungchi submarine canyons located near the Chiali paleo-high. Bright seismic amplitudes were often observed at the incised valley heads of the canyon systems. The shales located near the incised valley heads and deposited during flooding stage possess the highest biogenic gas generation potential, as canyon fill reveals the second highest. Due to the high sediment accumulation rate in the foreland basin, organic matter in such a depositional environment tends to become diluted. A Class III AVO anomaly, inverted impedance lower than 4.7 e + 6 kg/M³*M/S and A/B (the ratio between the target horizon amplitude and the RMS amplitude from the background strata) greater than 1.78 were identified as valid direct gas indicators as sand is buried shallower than 1000 m. Class IV AVO anomaly and A/B greater than 1.4 were concluded to be the indicators of gas sand in the case that sand is buried deeper than 1600 m. Based on the results of sequence stratigraphic interpretation and the consistency between spatial geometries of seismic attributes and those of the submarine canyons, a stratigraphic trap associated with the incised valley heads was concluded to be the original gas entrapment style of the Hsinying and the Kuantian gas fields. Biogenic gas migrated after being trapped startigraphically, hence contributing to the present-day gas field structure. Due to the prevalent erosional features of the submarine canyons on the time structural maps, different types of stratigraphic traps formed in combination with faults and submarine canyons can be recognized easily.     

Understanding shallow gas occurrences in the Gulf of Lions

New coring data have been acquired along the western Gulf of Lions showing anomalous concentrations of methane (up to 95,700 ppm) off the Rhône prodelta and the head of the southern canyons Lacaze-Duthiers and Cap de Creus. Sediment cores were acquired with box and kasten cores during 2004–2005 on several EuroSTRATAFORM cruises. Anomalous methane concentrations are discussed and integrated with organic carbon data. Sampled sites include locations where previous surveys identified acoustic anomalies in high-resolution seismic profiles, which may be related to the presence of gas. Interpretation of the collected data has enabled us to discuss the nature of shallow gas along the Gulf of Lions, and its association with recent sedimentary dynamics. The Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. Away from the flood-related sediments off the Rhône delta, the organic matter is being reworked and remineralized on its way along the western coast of the Gulf of Lions, with the result that the recent deposits in the canyon contain little reactive carbon. In the southernmost canyons, Lacaze-Duthiers and Cap de Creus, the gas analyses show relatively little shallow gas in the core samples. Samples with anomalous gas (up to 5,000 ppm methane) are limited to local areas where the samples also show higher amounts of organic matter. The anomalous samples at the head of the southern canyons may be related to methanogenesis of recent drape or of older sidewall canyon infills.     

Hydrocarbon accumulation model for Neogene traps in the Chengdao area,Bohai Bay Basin,China

Chengdao is an offshore area in the Bohai Bay Basin that contains approximately 25.7 × 10⁸ bbl of oil and gas reserves within the sandstone reservoirs in Neogene strata. However, previous predictions of hydrocarbon accumulation in Neogene traps are inaccurate, resulting in a current failure rate of 50% when drilling for hydrocarbons in this area. To build an improved exploration model for Neogene traps, we select 92 traps from Neogene strata in the Chengdao area to quantify the filling degree, which is an indicator of hydrocarbon accumulation efficiency. The quantified filling degree is based on actual geological and exploration data and differs significantly among various trap types. The filling degree of traps also varies significantly with their structural locations and decreases generally from the northwest to the southeast along the Chengbei Fault zone. Vertically, the filling degree is highly heterogeneous, initially increasing from the bottom to the middle of Neogene strata and then decreasing towards the top of the strata. These Neogene hydrocarbon reservoirs are sourced from the Paleogene, and as they lay vertically away from the source rocks, their hydrocarbon enrichment is constrained largely by hydrocarbon migration distance and vertical migration pathways. The sealing capacity of faults and cap rocks, sandbody orientation and reservoir sedimentary facies determine the maximum column height, which in turn affects the amount of hydrocarbon accumulation within these traps. A scatter plot analysis of individual controls and volumetric filling for each trap type is compiled using multivariate linear regression analysis to quantify controls and the dominant control of hydrocarbon accumulation is determined.