Platform Margin Slope Research Articles

Prediction of Permian karst reservoirs in the Yuanba gas field, northern Sichuan Basin, China

Karst reservoir has great hydrocarbon potential, however karst reservoir prediction is inhibited by the strong lateral and vertical heterogeneity of karstification which in turn results in recognition difficulty when geological and geophysical methods are used in isolation. Combined geological and geophysical methods, including core observation, thin section analysis, well log interpretation, seismic attributes and seismic inversion are applied to understand the depositional environment, types of karstification, the geophysical response and distribution of karst reservoir and the controls over and evolution model of karst reservoir in Permian Yuanba gas field, northern Sichuan Basin. The results show that there are four types of major seismic facies recognized in the study area, which correspond to platform margin reef, carbonate platform, platform margin inter-bay and platform margin slope. Karstification can be divided into three zones: the supergene karst zone, the vertical seepage zone and the horizontal underflow zone, among which the supergene karst zone have the strongest karstification. Karstification is highlighted with low seismic impedance, low Poisson's ratio, and high seismic attenuation on seismic inversion as well as micro scale paleo geomorphology recognized by trend surface analysis. By comparing the distribution of karstification with paleo geomorphology, fault distribution and gas contents, it can be observed that the karstification have a good matching relationship with these factors. The dominant type of karstification is epigenic karst, which is strongly influenced by depositional facies and paleo geomorphology. Fracture networks have contributed to karstification but are not the dominant factor of karst formation. The integration of geological and geophysical methods can predict karst reservoir with high accuracy within large area and can be applied to karst reservoir hydrocarbon exploration of similar geological setting.

Characteristics, genetic mechanism, and hydrocarbon accumulation of Cambrian gravity flow in the eastern Gucheng area, Tarim Basin, China

The platform margins of Gucheng and Luoxi in the Cambrian are developed on both sides of the eastern Tarim Basin. Based on the analysis of cores, thin sections, and seismic data of Luoxi platform margin slope areas, we systematically studied the formation condition, reservoir characteristics, and favourable accumulation factors of gravity flow. The types of gravity flow include debris flow, high‐density flow, and low‐density flow, and the high‐density flow reservoir is better. Hydrothermal fluids have an active effect on reservoir reconstruction. Six‐phase reefs are developed in the platform margin, and the lithology of the reservoir is algal framework dolomite and granular dolomite. Multi‐phase reefs form an inclined slope. The 3D seismic and geochemical data show that the platform margin reefs have karst unconformity characteristics. The platform margin slope has the requirements of geomorphology and reservoir source, and the total area of slope gravity flow is 5854 km2. The gravity flow in the Gucheng platform margin slope is near the Cheerchen fault zone, and tectonic movement forms the nose‐like structure. The gravity flow is between the source rocks of the Xidashan Formation and the Heituwa Formation. Hydrocarbon generation and caprock are both favourable for hydrocarbon accumulation. Multi‐phase events of hydrothermal fluids are favourable to the reservoir and hydrocarbon charging. Gravity flow is important for oil and gas exploration in the Tarim Basin.

Mechanism of formation and Characteristics of deposition of Cambrian gravity flow in the eastern gucheng area, Tarim Basin

The Gucheng Cambrian margin and Luoxi Cambrian margin is on both sides of eastern Tarim basin. Base on the analysis of cores,thin sections and seismic data of Luoxi slope areas,this paper systematically studies the formation condition, reservoir characteristics and favorable accumulation factors of gravity flow. The gravity flow sedimentary types include debris flow, high density flow and low density flow, debris flow and high density flow reservoir is better. Hydrothermal reconstruction has active effect on reservoir. There are six phases mound-beaches develop in platform margin, reservoir characteristics are algae framework dolomite and granular dolomite. Phases of mound-beaches develop to steep slope. Seismic and geochemical data indicate that the mound-beaches are exposed with corrosion characteristics,the platform margin-slope have the requirements of geomorphologie antique and quality reservoir source.

New insight into the source of metals in Hg deposits at the southwestern margin of the Yangtze Platform, China: Evidence from mercury stable isotope compositions

The genesis of the typical Hg-Au-Sb metallogenic district in the southwestern margin of the Yangtze Craton, Southwestern China, remains controversial. A systematic study of the mercury (Hg) concentrations and Hg isotopic compositions of the Hg deposit at Paiting was conducted to constrain the sources of metals. Mass-independent fractionation (MIF) of Hg isotopes with slight positive Δ199Hg (0.03‰ to 0.22‰) was observed in the Hg ore samples, indicating that Hg transported by the ore fluids was primarily derived from the Cambrian black shale as these rocks show similar Δ199Hg signatures (−0.02‰ to 0.08‰). Based on these newly obtained Hg isotope ratios, we propose that deep hydrothermal fluids promoted the mobilization of Hg in Hg-bearing strata (i.e., Cambrian Niutitang Formation black shale) in the paleo-basin sedimentary strata to form Hg-enriched fluids. The Hg-enriched fluids flowed and permeated from the lower to the upper strata during mineralization and were transported to the platform margin slope facies along faults, followed by Hg precipitation to form Hg deposits. Hg isotope ratios could be a robust tracer to distinguish the metal sources of hydrothermal Hg deposits.

Reconstruction and petroleum geological significance of lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in northwestern Sichuan Basin, SW China

Based on analysis of outcrop, drilling, logging and seismic data, and geotectonic background, the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed, and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed. The Maokou Formation is divided into 3 long-term cycles, namely LSC1, LSC2 and LSC3, which correspond to the Member 1, Member 2 and Member 3 of the Maokou Formation, respectively. Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting, the area had tectonic differentiation in NWW and NE, and sedimentary basement took on episodic settlement from north to south, as a result, the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf. According to the residual thickness, paleokarst geomorphologic units such as karst highland, karst slope and karst depression at different stages were reconstructed. The karst geomorphological units were developed successively on the basis of sedimentary geomorphology. Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation. The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations: high energy grain shoal and karst monadnock, platform margin slope and karst slope. Based on this understanding, the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology. The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.

Main factors for large accumulations of natural gas in the marine carbonate strata of the Eastern Sichuan Basin, China

The natural gas accumulation zone, where the marine carbonate rock strata are developed, was formed in the eastern Sichuan Basin under the influence of several main tectonic movements (Caledonian Movement, Indosinian Movement, Yanshanian Movement, and Himalayan Movement). Most natural gas reservoirs exhibit the structural-stratigraphic traps together with multistage accumulation, late-stage adjustment and reformation, et cetera. The natural gas accumulation zone (or so-called gas reservoir groups) is controlled by the following main factors: multi-sourced and multi-formed hydrocarbons for marine source rocks (i.e. Lower Silurian Longmaxi Formation, Lower Permian, Upper Permian Longtan Formation), paleo-uplift, paleoslope, and the hinge belt controlled by the steep dip structures, namely the Lower and Middle Triassic high-quality gypsum. Three sets of high-quality source rocks (i.e. S1l, P1, P2l) account for the abundant hydrocarbon supply for natural gas accumulation in the eastern Sichuan area, especially in the destructed oil reservoir formed earlier. The said destructed oil reservoir not only provides the preservation space for natural gas reservoir that will take place later, but it also provides the hydrocarbon source for thermal cracking of hydrocarbons and thermochemical sulfate reduction (TSR). Although the gas reservoirs in the eastern part of the Sichuan Basin experienced multi-stage adjustment and reformation at later times, the thick and high-quality gypsum as well as the mudstone, as available caprocks, have offered a good preservation condition for the underlying gas reservoirs. The paleohighs (e.g. Luzhou paleohigh and Kaijiang paleohigh), the Permian platform margin slope, and the structurally transformed slope under the function of the steep dip anticline in the eastern Sichuan not only form the high-quality carbonate reservoir, but they also became favorable for oil and gas accumulation. The difference in hydrocarbon generation history of the source rock and multistage accumulation of hydrocarbons in the eastern part of the Sichuan Basin caused the diversity in the location of the gas reservoirs, where the gas reservoirs with low H2S contents are distributed in the eastern Sichuan Basin (e.g. Jiannan gas field), while the gas reservoirs with high H2S content are located in the northeastern Sichuan Basin (e.g. Puguang gas field).

四川盆地中二叠统层序地层格架下岩相古地理特征

随着对四川盆地碳酸盐岩储层的重视和大力勘探，中二叠统发育的滩相储层也构成了盆内有利的含气层位之一。本文根据岩心观察、野外剖面实测、镜下岩石薄片鉴定分析和其它测试资料，结合岩石学、古生物、沉积构造等沉积相标志的综合研究，认为研究区中二叠世主要发育浅水型碳酸盐岩台地沉积，细分为碳酸盐岩台地和台地前缘斜坡相及若干亚相和微相。以经典层序地层学理论为指导，通过以岩性–岩相转换界面为主的层序界面识别，确定中二叠统发育5个II型三级层序，每个层序划分出TST和HST两个沉积体系域。在沉积相和层序地层学分析的基础上，以各层序体系域作为编图单元和“单因素分析综合作图法”为技术方法，编制了各体系域的岩相古地理图并对其演化特征进行了分析。结合各相带储集物性特征，认为台缘滩和台内滩为最有利储集相带，尤以SQ2-HST和SQ4-HST沉积期形成的滩相颗粒灰岩经过白云石化作用改造后更具储集意义。 With the recognition and vigorous exploration of carbonate reservoirs in Sichuan Basin, bank facies reservoir developed in middle Permian Maokou and Qixia Formation also constitute beneficial layer of the basin where new gas source can be discovered. According to core observation, field profile measurement, microscopically rock thin section identification, and other testing data, combining the research of sedimentary facies signs such as petrology, paleontology, sedimentary structure, etc., the area develops sedimentary model as shallow-water carbonate platform in middle Permian, further classifies as the sedimentary facies of carbonate platform, platform margin slope and several subfacies and microfacies. We apply Vail’s classical sequence stratigraphy theory as guidance and identify stratigraphic sequence boundaries as lithology-lithofacies convert interface, so determine that Middle Permian develops five Ⅱ-type third degree sequence, and each sequence is divided into two depositional system tracts as TST and HST. Base on sedimentary facies and sequence stratigraphy analysis, we take each sequence systems tract as the mapping unit and the “single-factor analysis and comprehensive mapping method” as the technical methods and compose lithofacies paleogeography map of each sequence system tract and describe its evolution character. Associated with reservoir physical property character of each phase belt, the platform edge shoals and inner platform shoals that are considered the most beneficial reservoir facies, especially shoal facies grain limestone in SQ2 HST and SQ4 HST which was dolomitisated has more reservoir significance.

Seismic sequence stratigraphy and sedimentary evolution of carbonate reservoirs in the Sinian Dengying Formation, Middle Sichuan Basin, Southwest China

The Sinian Dengying Formation in the Sichuan Basin, southwest China, mainly consisting of dolomites, is one of the most ancient gas-producing series in the world. During the past half-century, gas exploration in the formation has been largely based on the lithostratigraphic correlation, but a regional correlation scheme of time significance is usually insufficient, resulting in the difficulty of lateral correlation of strata between gas fields. Aiming to overcome the problem, we completed an interpretation of about 2500-km 2D regional seismic lines by using the seismic sequence analysis method. As a result, a sequence stratigraphic framework was successfully constructed, which consists of two sequences and five systems tracts. By integrating analysis of isopatch maps with stratal stacking patterns, we identify three depositional facies belts within the formation, which are a shallow-water platform facies belt in the eastern and southern regions, a relatively deep-water (intraplatform) basin facies belt in the northwestern region, and a northwest-dipping slope facies belt between them. During the development of sequence one in the lower of the Dengying Formation, retrogradation and aggradation dominated in the eastern and southern platform region whereas depositional condensation prevailed in the northwestern basin region. At that time, the depocenter was located on the eastern and southern platform region. However, sequence two in the upper of the Dengying Formation is dominated by the northwest-dipping sigmoid, oblique and shingled prograding packages of the platform-margin slope facies belt, indicating that the depositional center was shifted to the previous basin region in the northwest. As a result, the basin was filled gradually, and the platform-slope-basin topography was finally evolved into a northwest-dipping ramp. Our study suggests that the Late Sinian Sichuan Basin would consist of a series of shallow-water platforms separated by relatively deep-water depressions or (intraplatform) basins, which provides important clues for gas exploration.

Reef types and sedimentation characteristics of Changxing formation in Manyue-Honghua Section of Kaixian, Northeastern Sichuan Basin

In Changxing (长兴) stage, Manyue (满月)-Honghua (红花) Section of Kaixian (开县), northeastern Sichuan (四川) Province was located in the platform-margin slope, which was the advantage area of reef-shoal depositional system developing. The strata of Changxing Formation are continuously exposed on both Honghua Section and Manyue Section. Four and two depositional cycles can be identified in Honghua Section and Manyue Section, respectively. They are all platform-margin reef-shoal deposits. Their lithologic associations have obvious sequence, which is as follows: micrite bioclast limestone, bafflestone, bindstone, framestone, and bioclast limestone, in order from below. The paleontological assemblages are controlled by water depth and genetic facies. Six paleontological assemblages can be identified in Honghua Section and Manyue Section; they are coral-calcareous algae-calcareous sponge assemblage, calcareous sponge-calcareous algae assemblage, calcareous sponge-calcareous algae-coral-bryozoan assemblage, calcareous sponge-calcareous algae-hydra assemblage, calcareous sponge-calcareous algae-hydra-bryozoan assemblage, and calcareous sponge-calcareous algae-hydra-bryozoan-coral assemblage. The study of lithologic associations and paleontological assemblages indicates that the water body shallowed upward in both the growth cycle of a single reef and the higher-grade depositional cycle. According to the water depth types of reef (bioherm) developing, three different reef-shoal depositional systems can be identified in study area: deep-water-type bioherm, transitional-type reef (bioherm), and more shallow-water-type reef.

Devonian Frasnian-Famennian transitional event deposits of Guangxi, South China and their possible tsunami origin

A suite of event deposits, isochronous and interrelated in origin, occurs widely near the Frasnian-Famennian boundary in Guangxi, South China. It is mainly distributed in the facies areas of the platform-margin slope and inter-platform rift-trough. The rudstone or calcirudite occur in the Liujing section of Hengxian and Luoxiu and Baqi sections of Xiangzhou in the facies area of platform-margin slope. The turbidites are observed in the sections of Yangdi and Baisha of Guilin, Du’an of Debao, Nandong and Sanli of Wuxuan, Xiangtian, Ma’anshan of Xiangzhou, Nayi of Chongzuo, Yunpan of Shanglin in the facies area of inter-platform rift-trough. The massive homogenites occur in sections of Mangchang, Luofu and Road from Nandan to Tian’e and Ma’anshan etc in the facies area of inter-platform rift-trough. Herein event deposits can be correlated in stratohorizon to the turbidite in the bottom of the Lower triangularis Zone in Hony railroad cut of Belgium, Devils Gate of Nevada, USA, Atrous of Morocco, South Urals and Fore-Kolyma of northeastern Siberia of Russia, and erosional discontinuities and brecciation in the Frasnian-Famennian boundary in South Polish-Moravian shelf. The event deposits could be caused by a violent tsunami related to bolide impacts into ocean.

Biofacies Evidence for Late Cambrian Low-Paleolatitude Oceans, Western United States and Central Asia

Biofacies that formed on carbonate platform-margin slopes adjacent to an early Paleozoic, low-paleolatitude paleoocean are contained in the Upper Cambrian Swarbrick Formation, Tyby Shale, and Upper Cambrian-lowest Ordovician Hales Limestone of the Hot Creek Range, Nevada, and the Upper Cambrian-lowest Ordovician part of the Shabakty Suite of the Malyi Karatau, southern Kazakhstan. These in-situ limestones formed in platform-margin slope and basin-plain environments. Shoal-water faunal assemblages occur in carbonate-turbidite and debris-flow deposits interbedded with in-situ deeper water assemblages of the submarine-fan facies. Abundant sponge spicules, geographically widespread benthic trilobites, and rare ostracodes occur in some of the in-situ beds. In contrast, the shoal-water platform environments were well oxygenated and contain mainly endemic trilobite assemblages. These biofacies characteristics support an interpretation that Late Cambrian oceans were poorly oxygenated, but not anoxic, below the surface mixing layer and that benthic trilobite faunas were widely distributed in response to the more-or-less continuous deep water, low-oxygen habitats. Elements of the Late Cambrian low-oxygen biofacies are widespread in the Tien Shan structural belt of China and the Soviet Union, in central and eastern China, and along the western margin of early Paleozoic North America. This facies distribution pattern defines the transition from low-paleolatitude, shoal-water carbonate platformsmore » to open oceans which have since been destroyed by pre-Late Ordovician and pre-middle Paleozoic Paleotectonic activity.« less

Alternative Stratigraphic Interpretations for Thrust Sheets Beneath Southern Appalachian Crystalline Overthrust: ABSTRACT

In eastern Tennessee, strata as young as Mississippian lie directly beneath the Great Smoky thrust. There, a stack of three sheets (Pulaski, Saltville, and Copper Creek) contains an estimated thickness of 19,000 ft of sedimentary rocks above Precambrian basement. Seismic profiles obtained as part of the site study for the proposed Appalachian Ultra-deep Core Hole (ADCOH), 85 mi southeast of the Great Smoky fault, have been interpreted to reflect subhorizontal Paleozoic strate about 5,000 ft thick beneath overthrust crystalline rocks and above autochthonous crystalline basement. The most remarkable relationship to be explained is not that subhorizontal Paleozoic strate extend southeastward to the ADCOH site east of the Brevard zone, but rather by what mechanism such a great thickness of Paleozoic strata in several thrust sheets is lost southeastward. Six alternative scenarios of a structural cross section from eastern Tennessee to the ADCOH site show the proposed core piercing a single subhorizontal stratigraphic succession formed by (1) attached platform strata, (2) the Copper Creek thrust sheet, (3) the Saltville thrust sheet, (4) the Pulaski thrust sheet, (5) a previously unknown thrust sheet that came from the eastern platform margin slope of North America, and (6) a thrust sheet of terrane accretedmore » from another continent, probably Africa. The subhorizontal strata at the ADCOH site also may represent a stacking of thrust sheets, each containing only part of the stratigraphic thickness preserved farther northwest. Stratigraphic criteria will permit identification of a specific thrust sheet or of stacked thrust sheets in the core hole. The indicated southeastward thinning of strata in thrust sheets beneath the crystalline overthrust raises questions about foreland thrusting mechanisms.« less