Aiping Fan

Climatic and tectonic controls of lacustrine hyperpycnite origination in the Late Triassic Ordos Basin, central China: Implications for unconventional petroleum development

The Triassic Yanchang Formation contains the main oil-bearing strata in the Ordos Basin, central China. But the sedimentology of the Upper Triassic is still under debate, and flood-generated, hyperpycnal-flow deposits and their implications for unconventional petroleum development have long been overlooked. Our study indicates that hyperpycnites are well developed in the seventh oil member of the Yanchang Formation. They are characterized by couplets of upward-coarsening intervals and upward-fining intervals, separated by microscale erosion surfaces. The origination of hyperpycnal flows was controlled mainly by episodic tectonic movements and the humid climate. The deposits extend from distributary estuaries into the deep lake, have intercalations of dark shales and tuffs, and coexist with debrites and turbidites as a result of the progradation of subaqueous fans. The hyperpycnites have implications for unconventional petroleum reservoirs, because the flows supplied not only large amounts of coarse grains and organic material to the deep-water, fine-grained central lake sediments but also affected the ecosystems, resulting in a higher total organic carbon content in the sediments.

Depositional and Diagenetic Controls on Sandstone Reservoirs with Low Porosity and Low Permeability in the Eastern Sulige Gas Field, China

In order to determine the genesis and the factors that control the low-porosity and low-permeability sandstone reservoirs in the eastern Sulige Gas Field in the Ordos Basin, systematic studies on the sedimentary facies and diagenesis were conducted by means of analysis of cores, thin sections, fluid inclusions, X-ray diffraction, cathode luminescence and scanning electron microscope. It was found that the sand bodies of the major gas reservoirs in the Shan1 section (P1S1) and the He8 section (P2H8) were formed during the Permian as sedimentary facies such as braided-channel bars, braided-river channels and point bars of a meandering river. Four types of diagenetic facies developed subsequently: in order from the best to the poorest properties these are type A (weak compaction, early calcite cement-chlorite film facies), type B (moderate compaction, quartz overgrowth-feldspar corrosion-kaolinite filling facies), type C (strong compaction, late calcite cement-quartz corrosion facies) and type D (matrix filling and strong compaction facies). This diagenesis is undoubtedly the main reason for the poor reservoir properties of sandstone reservoirs, but the sedimentary facies are the underlying factors that greatly affect the diagenesis and thus the reservoir performance. Favorable diagenetic facies developed mainly in relatively small lithofacies such as braided-river channels, channel bars and point bars. The vertical distribution of the physical properties and the diagenetic facies of the reservoirs are related to the stratigraphic succession. Most of the sandstones between mudstones and thin beds of sandstone are unfavorable diagenetic facies. Analyses indicate that siliceous cementation can hardly be stopped by hydrocarbon filling. Authigenic chlorite could hardly protect the primary porosity. It not only occupies pore space, but also blocks pathways through sandstone reservoirs, so that it has significant influence on the permeability. Authigenic chlorite cannot be used as a marker for a specific sedimentary facies because it can be formed in different sedimentary facies, but it indicates high hydrodynamic conditions and presence of favorable reservoirs.

A Marine or Continental Nature of the Deltas in the Early Cretaceous Lingshandao Formation—Evidences from Trace Elements

A succession with well-developed soft-sediment deformation structures and well-exposed gravity-flow deposits in the Early Cretaceous Lingshandao Formation has caused considerable controversy on whether the nondisputed deltaic environment was marine or continental. This dispute cannot be solved by mere sedimentological analysis and, moreover, too few fossils are present to be decisive. The main objective of the present contribution is to shed light on this problem on the basis of trace-element analysis.

Soft-sediment deformation structures in cores from lacustrine slurry deposits of the Late Triassic Yanchang Fm. (central China)

Abstract The fine-grained autochthonous sedimentation in the deep part of a Late Triassic lake was frequently interrupted by gravity-induced mass flows. Some of these mass flows were so rich in water that they must have represented slurries. This can be deduced from the soft-sediment deformation structures that abound in cores from these lacustrine deposits which constitute the Yanchang Fm., which is present in the Ordos Basin (central China). The flows and the resulting SSDS were probably triggered by earthquakes, volcanic eruptions, shear stress of gravity flows, and/or the sudden release of overburden-induced excess pore-fluid pressure. The tectonically active setting, the depositional slope and the high sedimentation rate facilitated the development of soft-sediment deformations, which consist mainly of load casts and associated structures such as pseudonodules and flame structures. Sediments with such deformations were occasionally eroded by slurries and became embedded in their deposits.

Coexistence of Hyperpycnites, Debrites and Turbidites and Their Geological Significances of Unconventional Petroleum

The genesis of deep water sandstone has become the focus both of sedimentology research and petroleum industry (Etienne et al., 2012), however, people still know little about these deep water sediments flows and depositional mechanisms (Talling et a.,2012). Most of the deep water sandbodies were interpreted as turbidites in the past decades, until new theory of sandy debris flow challenged the classic theory of turbidity current (Shanmugam, 1996). What is more, new understanding of hyperpycnal flow fulfilled the theory of turbidity current (Mulder et al., 1995). Although arguments still exist among sedimentologists on origination of some deep water sandstone, relationships among sandy debris flows, turbidity currents and hyperpycnal flows were unanimously agreed to be the key scientific issue of deep water gravity flows and deep water sedimentology. Complex transformations and inductions not only exist among sandy debris flows, turbidity currents and hyperpycnal flows, but also hyperpycnites, debrites and turbidites co-occurs and co-exist in time and space usually. The variability and complexity of geological conditions in long geohistory determine the existence of multiple explanations of the same deposition products among sedimentologists.

Detailed sedimentary facies of a sandstone reservoir in the eastern zone of the Sulige gas field, Ordos Basin

Abstract Given sustaining exploration, the eastern zone of the Sulige gas field may soon become a key area of exploitation. In order to explore its genesis, types and distribution of the reservoir sandstones in the eastern zone of this gas field, we focused in our study on the provenance and detailed sedimentary facies of sandstone of the He8 (the eighth part of the Shihezi formation, Permian system) and Shanxi1 (the first part of the Shanxi formation, Permian system) members, based on core observations, analyses in petrography, granularity and logging. The results show that: 1) the sandstone provenance of Shanxi1 and He8 in the eastern zone of the Sulige gas field is from the north of the Ordos Basin, characterized by dual directions from the north and northeast. 2) The He8 and Shanxi1 members were deposited in a fluvial-delta sedimentary system. The He8 was mainly deposited in braided rivers, including braided channels, channel bars, levee and floodplain sub-environments, whereas the Shanxi1 Member was deposited in braided rivers and deltas, including braided channels, channel bars, floodplains, tributaries and inter-tributary sub-environments. 3) Sedimentary facies bands migrated in drastic fashion towards the basin from the Shanxi1 to the He8 Member. Base levels of sedimentation generally present a trend of small increases in-amplitude, large decreasing amplitudes and slow and gradual Increases. 4) The continuity of the reservoir sandbodies along the source direction is better than that perpendicular to the direction. Compared with Shanxi1, both dimensions and continuity of the sandbodies in He8 are better from which we conclude that it is the most favorable part of the reservoir.

Provenance of the lower Es2 in the Shanghe area of the Huimin sag

Abstract The provenance of the lower Es2 in the Shanghe area was determined from an approach incorporating analysis, elemental ratios, paleocurrent direction, the typomorphic characteristics of detritus and the distribution of conglomerate and gritstone in the peripheral basin. Typical elemental ratios characteristic of the sedimentary area were compared with those from the source areas as abstracted from sediments of the Huimin sag using cluster analysis. The results show that the distribution pattern focuses on Mg/Mn, Fe/K, Al/Na, Ba/Mn and Al/Mg. Mg/Mn is the highest ratio, from 25 to 45. This is similar to the pattern from the Precambrian. Furthermore, paleocurrent direction was used to determine provenance by examining the distribution of gritstone and the seismic progradational reflections. The research results indicate that the provenance is to the northwest in the Precambrian strata where the sand grain size is rough. To the east there is siltstone, fine sandstone and mudstone. This is significant for the exploration of oil and gas within the study area.

Remaining oil distribution in Ng33 bottom water reservoir of Lin 2-6 fault-block in Huimin depression and potential tapping in horizontal well

Abstract Oil reservoirs with secondary bottom water in Ng33 members (in Guantao formation, Paleogene system) of Lin2-6 fault block in Huimin depression (Bohai Bay Basin) have entered the late stage of ultra-high water-containing-exploitation. Oil exploitation from vertical wells is becoming more and more inefficient. The reservoir type, with water displacing oil and the remaining oil distribution are specifically studied in order to improve the efficiency of the recovery ratio. An integrated scheme for adjusting horizontal wells has been designed and the key technique of the scheme optimized. The study shows that: 1) the positive rhythm of fluvial depositional features is the internal cause of the flooding of oil reservoirs while water injection, injection-production patterns and accumulative petroleum production are the external causes; 2) oil-water driving patterns have transferred from edge water advancing to bottom-water-coning; distribution of the remaining oil mainly concentrates in the upper rhythm and top of the middle rhythm in Ng33 members; 3) a great deal of remaining oil is enriched in high positions of faults, in axes of tiny structures, in stagnation areas among water-injection wells and oil-wells and in tectonic saddle areas with sparse wells. Compared with vertical wells, horizontal wells have advantages such as high recovery, high off-take potential, high critical output, large controlling areas and long time of bottom-water breakthrough.