The Zhuhai Formation in Lufeng Depression is mainly deposited during a depression stage. In this paper, we employ cores, wireline logs, and seismic reflections to reveal sequence architecture and depositional evolution in target intervals. The results indicated that one second‐order sequence and two third‐order sequences were identified and sequence stratigraphic framework was established. The second‐order sequence was developed in response to base‐level falling, while third‐order sequences were mainly formed during base‐level rising. In addition, each third‐order sequence did not change too much in thickness. Then the shore deposits (shoreface and foreshore) were delineated, and the depositional characteristics were described within the framework with the help of related data. The shoreface and foreshore deposits within the sequence stratigraphic framework varied significantly in stacking patterns of wireline logs and cores. Based on comprehensive analysis, it can be concluded that base‐level changes played an important role in controlling sequence architecture. And lacustrine fluctuations derived from accommodation space changes exerted an essential effect on depositional evolution. This study provides significant insights into revealing the sequence architecture and depositional evolution and discussing hydrocarbon potential in the transition stage in the Zhuhai Formation.
An important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es3) in the Liuzan area, Nanpu Sag, Bohai Bay Basin, China. The Es3 can be divided into five third-order sequences (from base to top: Sq1, Sq2, Sq3, Sq4 and Sq5). Utilising well logs, cores and high-resolution 3-D seismic data, this study investigates the development characteristics and styles of sediment transport pathways in the steep-slope zone of the lacustrine rift basin. Two styles of sediment transport pathway are identified in the study area, including the faulted trough and fault slope-break zone. The faulted trough is divided into a single faulted trough, synthetic faulted trough and antithetic faulted trough. The fault slope-break zone is composed of synthetic fault slope-break zone and antithetic fault slope-break zone. The fan-delta plain and fan-delta front are recognised in the study area. The different styles of delivery conduits control depositional facies types and the spatial and temporal evolution of fan-delta depositional systems from Sq3 to Sq5. Based on paleo-geomorphological reconstruction, the dispersal pattern of the steep-slope zone system divided into a northern steep-slope system and an eastern steep-slope system is established. In Sq3, the northern steep-slope system develops large-scale mixed sandy–muddy fan-delta deposits derived from the NW. The eastern steep-slope system feeds the medium-scale gravel-rich fan-delta sediments. In Sq4, the large-scale mixed sandy–muddy fan-delta deposits develop from the NNE direction in the northern steep-slope system. The small-scale sand-rich fan-delta sediments are transported from the eastern steep-slope system. In Sq5, the large-scale mixed sand–mud fan-delta deposits are fed by both the northern steep-slope system and eastern steep-slope system. The strata stacking pattern exhibits a seesaw mode in the study area. Based on the analysis of the channel-belt thickness and the fan-scales, the paleogeomorphology of the drainage area in Sq3 and Sq4 and sediment supply in Sq5 alternatively control the sequence stratigraphic architecture and sediment partitioning in the sink area. The model proposed in this study may aid in the prediction of favourable reservoirs and good source–reservoir–seal development in lacustrine rift basin.
The sequence architecture and depositional evolution of Dongying Formation in the steep slope zone are mainly developed in response to tectonism. The comprehensive analysis of cores, wireline logs, and seismic volumes is employed to reveal the relationship among sequence architecture, depositional evolution, and tectonism. The results indicate that the whole Dongying Formation can be divided into three third‐order sequences based on regional unconformity along the basin margin and slope and its correlative conformity in the basin centre. In addition, seven fourth‐order sequences are further divided based on regression surfaces within third‐order sequences. The sequence architecture of different‐order sequences is discussed and revealed in terms of systems tracts, stratal thickness and distribution, and seismic reflections. Four genetic types of sedimentary facies including fan delta, sublacustrine fan, near‐shore subaqueous fan, and braided delta are identified according to core observations, stacking patterns of wireline logs, internal configuration, and external geometry of seismic reflections. The depositional evolution in Dongying Formation is related to tectonic intensity and tectonic evolution stage in the steep slope. The whole Dongying Formation is in the midst of a strike‐slip and rifting stage during its depositional period. When tectonic movements are intensively active, it is mainly characterized by fan delta and near‐shore subaqueous fan along the steep slopes and sublacustrine fan in the basin centre. However, the opposite case is dominated by fan delta and braided delta around and along the uplifts. In addition, the palaeogeomorphology and sediment supply also play an important role in controlling facies distributions in the steep slope zone. The subsidence rate resulted from tectonism controls the sequence architecture and depositional filling in terms of fault activities and accommodation space in the steep slope.
The seismic merging dataset, wireline logs, core photographs, thin sections, and other geological information were employed to investigate sequence architecture and reveal depositional evolution of Enping Formation in Lufeng Depression, Pearl River Mouth Basin. The results showed that one second‐order sequence and four third‐order sequences were identified based on the unconformities along basin margin/slope and correlative conformities in the basin centre. In addition, several associated sedimentary facies or microfacies (including braided channel deposits, subaqueous distributary channel deposits, beach‐bar deposits, and so on) that were recognized within sequence stratigraphic framework and depositional evolution were revealed by well tie–seismic correlations with seismic attributes extracted from the stratal slicing in terms of sand sections/units in Enping Formation. It was characterized by progradational process from seismic attributes in general that the delta deposits gradually retrograded. Through comprehensive analysis, it can be concluded that episodic tectonic movements and subsequent uplift played an important role in controlling sequence architecture. Sediment supply and lacustrine level fluctuations derived from accommodation space changes exerted an essential effect on depositional evolution. This study provides new and robust insights into understanding the sequence architecture, depositional evolution, and hydrocarbon exploration in Enping Formation. In addition to the significance for academia, study of sequence architecture and depositional evolution are of great importance for hosting potential hydrocarbon reserves, because they are frequently related to important and potential reservoirs. Consequently, investigating sequence architecture and depositional evolution are significant economically as well.
Hulun Lake is located in Northeast China and is a well‐exposed modern lacustrine rift basin. In this study, recent fieldwork has demonstrated that it was mainly filled by alluvial fan and fan delta deposits along the western escarpment, which were controlled by the Eerguna Fault Zones. However, it was filled by river‐deltaic and eolian deposits along the eastern slope. The distributions of the depositional systems were found to be controlled by their positions within the basin. The western escarpment was determined to have influenced the distributions of the alluvial fan and fan delta deposits. In contrast, the river‐deltaic and eolian deposits were controlled by the sediment supply, paleogeomorphology, and wind forces. The asymmetric geometry of the basin was determined to have resulted in different facies associations. First, a region of maximum sediment thickness and high‐gradient escarpment was found to be located near a normal fault foot (Eerguna Fault). Second, a region of a shallower and low‐gradient slope was located near the margin of a river‐lake transition. The depositional characteristics obtained from the prospecting trench indicated that the coarse deposits on the western side were mainly derived from unexpected flooding, gravity, and episodic flooding in an ascending order. In contrast, the fine deposits on the eastern side were mainly sourced from a meandering river. A clear understanding of controlling factors on facies associations can potentially reveal the sedimentary process and evolution of the depositional systems, as well as provide an important methodology for the prediction of facies in other sedimentary basins with similar characteristics.
The southwestern slope of Lufeng Depression, which is located in the central part of Zhu I Depression is a significant geomorphic unit due to its petroliferous discoveries and tectonic evolutions since early Palaeogene. Based on the comprehensive analysis of cores, wireline logs, seismic volumes, and other related geological and geophysical information, the sequence architecture and depositional evolution was analysed, and their responses to regional tectonic processes and base‐level changes were revealed. The results indicated that two second‐order sequences and six third‐order sequences defined by different types of sequence boundaries are recognized. The second‐order and third‐order sequences were bounded by regional erosional unconformities on the depression margin or slope and unconformities in the central depression with the assist of stacking patterns of wireline logs and depositional characteristics of cores. The second‐order sequence generally consists of two regional transgressive–regressive cycles which can be further subdivided into several local sub‐cycles equivalent to corresponding third‐order sequences which can be identified from wireline logs and cores. The palaeogeomorphology resulted from compound and complex tectonic events in the early Palaeogene plays an important and main role in controlling sequence architecture. In contrast, the base‐level changes largely controls the sequence architecture in the late Palaeogene. The depositional systems within the sequence stratigraphic framework are mainly composed of braided delta deposits and shore deposits, which respectively includes braided channel, subaqueous distributary channel, lacustrine shale, beach bar, maximum flooding surface, shoreface, and foreshore. The base‐level changes, which apparently originated from tectonic uplift and subsidence and sediment supply, controlled the regional transgressions and regressions in sedimentary facies distributions. The development of subsequence (what we call it sand units or sections) from 2,980 to 2,880 in Enping Formation in the Early Palaeogene is obviously influenced by a decrease in tectonic subsidence (rifting stage) and an increase in accommodation space (base‐level rising), while the case is opposite in subsequence 2,850 of Enping Formation. However, the subsequence in Zhuhai Formation in the late Palaeogene from 2,760 to 2,600 was formed in stable tectonic settings (transition stage), and thereby, it was mainly dominated by base‐level changes which were caused by sediment supply and accommodation space changes.
The Tarim Basin, located in Western China, is a large sedimentary basin with a long geological evolution history stretching from the Sinian (Ediacaran) to the Neogene period. An enormous deep‐water gravity flow system, up to 3,000–5,000 m in thickness, developed during the Late Ordovician in the central and northern parts of the basin, with the great potential for forming large lithologic‐stratigraphic oil/gas traps. However, the poorly‐understood reservoir condition has caused oil/gas exploration to fall to a standstill. Based on several seismic surveys, well profiles, and outcrop data, this study focuses on sequence‐depositional systems, paleo‐geomorphology, and paleo‐geography of the Late Ordovician reservoir rocks. The goal is to probe into the relationship between different types of deep‐water gravity flow systems and the paleo‐geomorphology, taking the tectonic development into account. Three different depositional models are applied to interpret the deep‐water gravity flow systems preserved in the Upper Ordovician succession. The latter are distributed in the Tabei, Tazhong, and Tadong areas underneath the great deserts, respectively. During the Late Ordovician, the Tazhong area seemed to be a debris‐flow‐dominated setting with shallow, sandy channelized systems controlled by multiple slopes. The Tabei area was a debris‐flow‐dominated setting with few U‐shaped deep incised ditches filled with muddy debris, and the Tadong area a typical submarine turbidite‐dominated fan system in that time. These depositional models also attempt to predict the favourable reservoir sand bodies and lithologic‐stratigraphic oil/gas traps with the Upper Ordovician successions in the subsurface of the Tarim Basin.
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