Shiqiang Xia

Horsetail faults and rotational faults, which maintain slight differences, are all important identity markers for strike-slip movement. However, it is difficult to explain why horsetail faults also appear in the extensional regime. Using the Linnan Sag as a reference, physical experiments are conducted to reproduce the formation processes of horsetail faults in the extensional regime. Characteristics of these horsetail faults include: (1) horsetail faults are the product of strain superposition during two phases of noncoaxial extension; (2) preexisting faults generated in the initial stage may reactivate by the combined elongation of dip-slip motion, followed by oblique-slip faulting at the middle-late stage, thus becoming the principal fault of the horsetail faults; this initial distance is obviously less (approximately 46%) than its distance during the middle-late stage; (3) a large number of new dip-slip faults appear at the tip of the principal fault in the middle-late stage, spatially connecting with the principal fault to form the horsetail faults; and (4) other preexisting faults that intersect the principal fault are not necessary for forming the horsetail faults but their presence may make the principal fault prone to segmentation. Therefore, although horsetail faults in the extensional and strike-slip regimes are similar in shape, their formation conditions and fault properties are very different. Understanding the characteristics of the horsetail faults in the extension regime contributes to further study related to oil and gas migration and accumulation in the Petroliferous basin.

Based on seismic interpretation of 18 regional 3D high‐resolution seismic sections across Liaodong Bay, extension amounts, extension rates and extension coefficients of different seismic sections from the south to the north are calculated by employing balanced section technique. Then, spatio‐temporal distribution of extension amounts is characterized and interpreted. Finally, the analysis of genetic mechanism is conducted and analysed. The results indicate that tectonic evolution in the Paleogene of Liaodong Bay is characterized by episodic extension. Furthermore, spatio‐temporal distribution of extension amounts is relatively uneven. In terms of space, extension amounts are mainly caused by extensional displacement of main depression‐controlled boundary faults and syn‐depositional faults in the depression. In addition, extensional amounts along main depression‐controlled boundary faults are characterized by larger displacement. In terms of time, the periods for horizontal extensional movement can be divided into three stages: The Palaeocene‐Early Eocene (Kongdian Formation‐third member of Shahejie Formation), Middle‐Late Eocene‐Early Oligocene (first and second member of Shahejie Formation) and Middle‐Late Oligocene (Dongying Formation). Among them, extension during the Palaeocene‐Early Eocene occurs both in the north and south of Liaodong Bay. However, extension intensity in the north is stronger than that in the south. Extension intensity in the middle and north during Middle‐Late Eocene‐Early Oligocene obviously increases. In contrast, it is dominated by strike‐slip movement during depositional period of Dongying Formation. Therefore, extension intensity is characterized by significant inheritance. Spatial–temporal distribution of extension amounts is predominantly controlled by tectonic evolution and associated tectonic deformation.

The Ordos Basin, as one of the most petroliferous basins in China, hosts 12.9 billion tons of oil reserves and 15.2 trillion cubic meters of natural gas reserves. The extensive hydrocarbon exploration and production shows that the southeastern Ordos Basin contains considerable potential in the Shanxi Formation and Lower Shihezi Formation. In this study, cores, wireline logs, thin sections, and heavy minerals are employed to carry out research on classification of sequence stratigraphy, identification of sedimentary facies, reconstruction of palaeo‐geomorphology, analysis of sediment supply, palaeo‐climate, and accommodation space changes. The results indicate that palaeo‐geomorphology gradient tends to be steep, which results in increasing hydrodynamics and turns to be wide and flat which results in decreasing accommodation space. In addition, heavy mineral distribution reveals the direction of sediment supply which indicates provenance changes. The palaeo‐climate changes commonly cause differences in lithology associations, monsoon and vegetation properties. The differences in temperature and humidity result in different colours of lithology associations which indicate various sedimentary environments. The prevalence of monsoon assists in the preservation of fluvial delta by controlling lateral migration of rivers through affecting vegetation and lithology associations. The accommodation space variations control the development of fluvial delta and result in various depositional characteristics of fluvial delta. All these factors trigger the transition from meandering river delta to braided river delta. The reconstruction of the transition and analysis of controlling factors play an important role on understanding the sedimentary evolution of analogous reservoir successions.

The solar radiation or insolation at a specific locality on the Earth's surface is determined by the distance of this point from the sun and by the angle with which the radiation hits the earth's surface. The theory that variations in the earth's orbit determine the amount of radiation and therefore the climate which is the Milankovitch Cycle. In non‐marine lacustrine rift basin, the changes of climate can be recorded by the palaeontology, and it also plays an important role in controlling lacustrine fluctuations. The lacustrine fluctuations are indicated by coastal onlap, which is the landward or lake‐ward onlap of non‐marine coastal deposits. In this paper, based on borehole data, wireline logs, seismic datasets, biostratigraphic data and palaeontology, the lacustrine fluctuation curve can be reconstructed from the onlap of lacustrine deposits in non‐marine sequences. The durations and magnitudes of these changes can be used to construct charts showing cycles of the relative rising and falling of lake level. In addition, the changes of lake level can also be reflected by the palaeontology such as pollen. The living conditions and distributions of pollen can be a good indicator for climate changes and lacustrine fluctuations in an a non‐marine setting. The calculation of the Milankovitch Cycle can be determined by the GR log in the study area which keeps a record of cycle changes. All climatic changes and lacustrine fluctuations are mainly resulted from the Milankovitch Cycles, and therefore, through the reconstruction of lacustrine fluctuations, climate changes and the Milankovitch Cycle, the correspondences among them can be clearly addressed. All the results can contribute a lot in theory research in the study area and lead to a new focus in practical hydrocarbon exploration and exploitation.

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.