Facies Architecture Model of the Shimentan Formation Pyroclastic Rocks in the Block-T Units, Xihu Sag, East China Sea Basin, and its Exploration Significance

A gas field consisting of volcanic reservoir rocks was discovered in the block-T units of the Xihu Sag,East China Sea Basin.The lithology of the volcanic rocks is dominated by tuff and reworked tuff.The lithofacies are dominated by base surge deposits of explosive facies.As the architecture model of volcanic facies is still uncertain,it has restricted the exploration and development of mineral resources in this area.Using core and cuttings data,the lithology,lithofacies,geochemistry as well as grain size characteristics of volcanic rocks were analyzed.Based on these analyses,the volcanic rocks in the well section are divided into three eruptive stages.The transport direction of each volcanic eruption is analyzed using crystal fragment size analysis.The facies architecture of the block-T units was established based on the reconstruction results of paleo-geomorphology.The results show that the drilling reveals proximal facies(PF)and distal facies(DF)of the volcanic edifices.However,the crater-near crater facies(CNCF)are not revealed.Compared with the reservoirs of the Songliao Basin,it is shown that the volcanic rocks in the Xihu Sag have good exploration potential;a favorable target area is the CNCF near the contemporaneous fault.

Sedimentary Facies Architectures of Third-Order Sequences from Early to Middle Triassic in Nanpanjiang Basin

There are complex and regular changes on sedimentary facies from the Early to the Middle Triassic in the Nanpanjiang basin. After the obvious drowned event of carbonate platforms in the transitional period between Permian and Triassic, carbonate platforms have evolved into the ramp type from the rimmed shelf type. The differentiation of sedimentary facies becomes clearer in space, which are marked by the changes from an attached platform to a turbidity basin and several isolated platforms in the basin. The striking characteristics are the development of oolitic banks on isolated platforms in Nanning and Jingxi and the reef and bank limestones in the margin of the attached platform in the Early Triassic. Despite the difference of the time span and the architecture of facies succession of third order sedimentary sequences, the process of the third order relative sea level changes reflected by the sedimentary facies succession of the third order sequences is generally synchronous. Therefore, six third order sequences could be discerned in the strata from the Early to the Middle Triassic in the Nanpanjiang basin. Using two types of facies changing surfaces and two types of diachronisms in stratigraphic records as the key elements, the sedimentary facies architectures of the third order sequences that represent sequence stratigraphic frameworks from the Early to the Middle Triassic in the Nanpanjiang basin could be constructed.

Lithofacies architecture and palaeogeography of the Late Paleozoic glaciomarine Talchir Formation,Raniganj Basin,India

Talchir Formation （Permo-Carboniferous） of the Gondwana Supergroup records the Late Paleozoic glaciation in Peninsular India. Talchir sedimentary succession of the Raniganj Basin, Damodar Valley Coalfields, Peninsular India, bears ten facies types grouped under three facies associations, viz., the proglacial conglomerate-sandstone facies association （CS）, the foreshore-shoreface conglomerate-sandstone-mudstone facies association （CSM） and the prodelta-shelf sandstone-mudstone facies association （SM）. Overall facies architecture reflects initial ice-covered terrestrial subgiacial sedimentation, which was subsequently reworked and emplaced subaqueously in front of the ice-grounding line, and finally overlapped by storm-laid prodelta-shelf sediments. Repeated glacial advance-retreats with shifts in the position of the ice-grounding line during phases of climatic amelioration led to multiple deglaciation-related fining-up cycles. Decoupled ice sheet and floating icebergs contributed icerafted debris （IRD） to these sediments. Gradual retreat of the ice sheet, however, restricted the supply of IRD towards top of the succession. Overlap of wave-agitated shoreface-shelf sediments on the glaciogenic sediments indicates widespread marine transgression caused by glacier melting during ice-house to green-house climatic transition, and crustal downsagging related to giacioisostasy. Subsequently, complete disappearance of the ice sheet caused basinal exhumation along with crustal uplift due to isostatic rebound, leading to multiple horst-graben bounded basinal systems, which received post-Talchir coal-bearing Gondwana sediments.

Evolution of the Paleogene succession of the western Himalayan foreland basin

The Paleogene succession of the Himalayan foreland basin is immensely important as it preserves evidence of India-Asia collision and related records of the Himalayan orogenesis.In this paper,the depositional regime of the Paleogene succession of the Himalayan foreland basin and variations in composition of the hinterland at different stages of the basin developments are presented.The Paleogene succession of the western Himalayan foreland basin developed in two stages,i.e.syncollisional stage and post-collisional stage.At the onset,chert breccia containing fragments derived from the hanging walls of faults and reworked bauxite developed as a result of erosion of the forebulge. The overlying early Eocene succession possibly deposited in a coastal system,where carbonates represent barriers and shales represent lagoons.Up-section,the middle Eocene marl beds likely deposited on a tidal flat.The late Eocene/Oligocene basal Murree beds,containing tidal bundles,indicate that a mixed or semi-diurnal tidal system deposited the sediments and the sedimentation took place in a tidedominated estuary.In the higher-up,the succession likely deposited in a river-dominated estuary or in meandering rivers.In the beginning of the basin evolution,the sediments were derived from the Precambrian basement or from the metasediments/volcanic rocks possessing terrains of the south.The early and middle Eocene（54.7-41.3 Ma） succession of the embryonic foreland possibly developed from the sediments derived from the Trans-Himalayan schists and phyllites and Indus ophiolite of the north during syn-collisional stage.The detrital minerals especially the lithic fragments and the heavy minerals suggest the provenance for the late Eocene/Oligocene sequences to be from the recycled orogenic belt of the Higher Himalaya,Tethyan Himalaya and the Indus-suture zone from the north during post-collisional stage.This is also supported by the paleocurrent measurements those suggest main flows directed towards southeast,south and east with minor variations.This implies that the river system stabilized later than 41 Ma and the Higher Himalaya attained sufficient height around this time.The chemical composition of the sandstones and mudstones occurring in the early foreland basin sequences are intermediate between the active and passive continental margins and/or same as the passive continental margins.The sedimentary succession of this basin has sustained a temperature of about 200 C and undergone a burial depth of about 6 km.

Evolution of Neoproterozoic siliciclastic Kerur Formation in the light of sequence stratigraphic framework: Badami Basin, Karnataka, India

Field-based sedimentology,state of the art facies analysis and sequence stratigraphic framework analysis have revealed the controls of local and global tectonics,basin-marginal slope,climate and changes in relative sea level (RSL) over the sedimentation pattern and evolution of a Neoproterozoic Kerur Formation within the Badami Group of Kaladgi Supergroup in India.The entire succession shows three major cycles of deposition.Facies study and fluvial architectural elemental analysis suggest considerable variations in depositional environments as well as palaeogeography.A transition from basin-margin alluvial cone deposits to braided system,initially with fluctuating ephemeral flows then to a steadier semi-perennial nature,is discernible within the 1^(st) cycle,in response to decreasing depositional slope with rising water table.The initial alluvial cone and braided ephemeral streams of high slope areas is designated as a product of low accommodation systems tract (LAST),while the semi-perennial system with steadier flows,representing the axial river of the initial rift valley,appears to be a product of high-accommodation systems tract (HAST).The 2^(nd) cycle begins with a perennial and steady braided river system and grades upward to a shallow marine succession,comprising wave-dominated,well-sorted sandstone,with a granular transgressive lag at the base.Thus,the bottommost fluvial interval of the 2^(nd) cycle constitutes the lowstand systems tract (LST).The marine succession represents deposits of outer shelf offshore to foreshore-beach settings and is composed of an initially deepening and fining upward transgressive systems tract (TST),followed by a coarsening and shallowing upward highstand systems tract (HST) with a maximum marine flooding surface (MFS) in between,demarcated by a shale-rich condensed zone.The 3^(rd) cycle,with its prograding alluvial fan and aggrading braided fluvial deposits and restricted occurrence,represents only the low accommodation systems tract(LAST) with a subaerial unconformity at the base.The basin evidently initiated in the western sector,followed by its eastward expansion during the first major rejuvenation of the basin margin faults,after the deposition of the 1^(st) cycle.After the basin-wide deposition of the 2^(nd) cycle,restricted development of the 3^(rd) cycle took place in the western sector only,following the second major rejuvenation of the fault system.The proposed sedimentological model,supported by established geochronological constraints,suggests that the sedimentation in the 1^(st) cycle begins with scree cones,alluvial fans and braided ephemeral channel networks,originated from faulted basin margins within a riftogenic setting possibly related to the global-scale extensional tectonics of Rodinia breakup.After the expansion of the basin,the marine inundation has been correlated to the transgression that possibly took place during the post-rift maturation stages.