Distribution Patterns of Calcite Cement in the Turbidite Sandstones of the Shahejie Formation in the Dongying Depression,Eastern China

Calcite cements are volumetrically dominant among the most diagenetic constituents in turbidite sandstones of the Paleogene Shahejie Formation in the Dongying Depression.The results show carbonate cements mainly occur in three phases:Calcite I(pre-compaction),Calcite II(post-compaction)and ankerite in the turbidite sandstone.The isotopic composition of Calcite I and Calcite II range from−4.3‰to 4.4‰,−3.5‰to 4.3‰PDB,respectively,suggesting that the calcite cements are mainly derived from the interbedded shales.The early calcite cement precipitated from the mixing of lacustrine and meteoric waters at 38-69℃.The late-phase calcite precipitated from pore waters modified by water-rock interactions at 64-126℃.Two distribution patterns of calcite cements can be identified,depending on their position within the sandstone.The peripheral cementation of the sandstones is near the sand-shale contact.The development of cement is controlled by sandstone thickness and the properties of the interbedded shale.For scattered cementation,the distribution of the calcite cement is not controlled by the sand-shale contact.Calcite cement was preferentially distributed in coarse-grained sandstones,indicating that sandstones with high original permeability are the preferred pathway for the migration of early diagenetic fluids.

Petroleum exploration and production in Brazil:From onshore to ultra-deepwaters

The Santos Basin in Brazil has witnessed significant oil and gas discoveries in deepwater pre-salt since the 21^(st)century.Currently,the waters in eastern Brazil stand out as a hot area of deepwater exploration and production worldwide.Based on a review of the petroleum exploration and production history in Brazil,the challenges,researches and practices,strategic transformation,significant breakthroughs,and key theories and technologies for exploration from onshore to offshore and from shallow waters to deep-ultra-deep waters and then to pre-salt strata are systematically elaborated.Within 15 years since its establishment in 1953,Petrobras explored onshore Paleozoic cratonic and marginal rift basins,and obtained some small to medium petroleum discoveries in fault-block traps.In the 1970s,Petrobras developed seismic exploration technologies and several hydrocarbon accumulation models,for example,turbidite sandstones,allowing important discoveries in shallow waters,e.g.the Namorado Field and Enchova fields.Guided by these models/technologies,significant discoveries,e.g.the Marlim and Roncador fields,were made in deepwater post-salt in the Campos Basin.In the early 21^(st)century,the advancements in theories and technologies for pre-salt petroleum system,carbonate reservoirs,hydrocarbon accumulation and nuclear magnetic resonance(NMR)logging stimulated a succession of valuable discoveries in the Lower Cretaceous lacustrine carbonates in the Santos Basin,including the world-class ultra-deepwater super giant fields such as Tupi(Lula),Mero and Buzios.Petroleum development in complex deep water environments is extremely challenging.By establishing the Technological Capacitation Program in Deep Waters(PROCAP),Petrobras developed and implemented key technologies including managed pressure drilling(MPD)with narrow pressure window,pressurized mud cap drilling(PMCD),multi-stage intelligent completion,development with Floating Production Storage and Offloading units(FPSO),and flow assurance,which remarkably improved the drilling,completion,field development and transportation efficiency and safety.Additionally,under the limited FPSO capacity,Petrobras promoted the world-largest CCUS-EOR project,which contributed effectively to the reduction of greenhouse gas emissions and the enhancement of oil recovery.Development and application of these technologies provide valuable reference for deep and ultra-deepwater petroleum exploration and production worldwide.The petroleum exploration in Brazil will consistently focus on ultra-deep water pre-salt carbonates and post-salt turbidites,and seek new opportunities in Paleozoic gas.Technical innovation and strategic cooperation will be held to promote the sustainable development of Brazil's oil and gas industry.

Formation of Hengchun Accretionary Prism Turbidites and Implications for Deep-water Transport Processes in the Northern South China Sea

Located at the end of the northern Manila Trench,the Hengchun Peninsula is the latest exposed part of Taiwan Island,and preserves a complete sequence of accretionary deep-sea turbidite sandstones.Combined with extensive field observations,a’source-to-sink’approach was employed to systematically analyze the formation and evolutionary process of the accretionary prism turbidites on the Hengchun Peninsula.Lying at the base of the Hengchun turbidites are abundant mafic normal oceanic crust gravels with a certain degree of roundness.The gravels with U-Pb ages ranging from 25.4 to23.6 Ma are underlain by hundreds-of-meters thickness of younger deep-sea sandstone turbidites with interbedded gravels.This indicates that large amounts of terrigenous materials from both the’Kontum-Ying-Qiong’River of Indochina and the Pearl River of South China were transported into the deep-water areas of the northern South China Sea during the late Miocene and further eastward in the form of turbidity currents.The turbidity flow drastically eroded and snatched mafic materials from the normal South China Sea oceanic crust along the way,and subsequently unloaded large bodies of basic gravel-bearing sandstones to form turbidites near the northern Manila Trench.With the Philippine Sea Plate drifting clockwise to the northwest,these turbidite successions eventually migrated and,since the Middle Pleistocene,were exposed as an accretionary prism on the Hengchun Peninsula.

Structural characteristics and deep-water hydrocarbon accumulation model of the Scotian Basin, Eastern Canada

Commercial hydrocarbon reservoirs have been discovered in shallow-water areas of the Scotian Basin, Eastern Canada. However, knowledge about the structure and hydrocarbon accumulation characteristics of the basin is still insufficient, which constrains the oil and gas exploration in deep-water areas. Based on comprehensive data of magnetic anomalies, seismic survey, and drilling, this study determines the structure characteristics of the Scotian Basin and its hydrocarbon accumulation conditions in deep waters and evaluates the deep-water hydrocarbon exploration potential. The transform faults and basement structures in the northern basin control the sedimentary framework showing thick strata in east and thin strata in west of the basin. The bowl-shaped depression formed by thermal subsidence during the transitional phase and the confined environment (micro basins) caused by salt tectonics provide favorable conditions for the development of source rocks during the depression stage (also referred to as the depression period sequence) of the basin. The progradation of large shelf-margin deltas during the drift phase and steep continental slope provide favorable conditions for the deposition of slope-floor fans on continental margins of the basin. Moreover, the source-reservoir assemblage comprising the source rocks within the depression stage and the turbidite sandstones on the continental margin in the deep waters may form large deep-water turbidite sandstone reservoirs. This study will provide a valuable reference for the deep-water hydrocarbon exploration in the Scotian Basin.