Sandy Debris Research Articles

Genetic mechanisms of high-quality tight siliciclastic reservoirs: A case study from the Upper Triassic Xujiahe Formation in the Yuanba area, Sichuan Basin, China

This study analyzed the petrological characteristics, diagenesis, pore types, and physical properties of the tight coarse-grained siliciclastic sequences in the third member of the Upper Triassic Xujiahe Formation (also referred to as the Xu-3 Member) in the western Yuanba area in the northeastern Sichuan Basin, China, based on the results of 242.61-m-long core description, 292 thin-section observations, scanning electron microscopy (SEM), and 292 physical property tests. The types and genetic mechanisms of high-quality tight coarse-grained siliciclastic reservoirs in this member was determined thereafter. The research objective is to guide the exploration and development of the tight coarse-grained siliciclastic sequences in the Xu-3 Member. The results of this study are as follows. Two types of high-quality reservoirs are developed in the coarse-grained siliciclastic sequences of the Xu-3 Member, namely the fractured fine-grained sandy conglomerate type and porous medium-grained calcarenaceous sandstone type. Hydrodynamic energy in the sedimentary environment is the key factor controlling the formation of high-quality reservoirs. These high-quality reservoirs are developed mainly in the transitional zone with moderately high hydrodynamic energy between delta-plain braided channels and delta-front subaqueous distributary channels. The dolomitic debris (gravel) content is the main factor affecting the reservoirs’ physical properties. The micritic algal debris and sandy debris in the dolomitic debris (or gravels) tend to recrystallize during burial, forming intercrystalline pores within. In the medium-grained calcarenaceous sandstones, intercrystalline pores in the dolomitic debris are formed at the early diagenetic stage, and a pore system consisting of structural fractures connecting intergranular pores, intergranular dissolution pores, and kaolinite intergranular micropores is developed at the late stage of diagenesis. The formation of intercrystalline pores in dolomite gravels and gravel-edge fractures, a pore system connected by gravel-edge and tectonic fractures, is closely related to the dolomite gravels in the sandy fine-grained conglomerates.

Sedimentary facies analysis of the fluvial environment in the Siwalik Group of eastern Nepal: deciphering its relation to contemporary Himalayan tectonics, climate and sea-level change

The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be deposited in the fluvial environment and controlled by contemporary Himalayan tectonics and climate. In this study, we established the fluvial environment and its controlling factors responsible for the deposition of the Siwalik succession along the Muksar Khola section in the eastern Nepal Himalaya. Five sedimentary facies associations are identified; these are interpreted as the deposits of flood plain-dominated fine-grained meandering river (FA1), flood-dominated overbank environment (FA2), sandy meandering river (FA3), anastomosing river (FA4), and debris flow-dominated gravelly braided river (FA5). These changes in the fluvial system occurred around 10.5 Ma, 10.0 Ma, 5.9 Ma and 3.5 Ma, defined by existing magnetostratigraphy constraints, due to the effects of hinterland tectonics, climate and sea-level change and continuous drifting of the foreland basin towards the hinterland concerning depositional age. The thick succession of an intraformational conglomerate reveals intensification of the monsoon started around 10.5 Ma in the eastern Nepal Himalaya. The present study also shows asynchronous exhumation of the Himalaya from east to west brought a significant difference in the fluvial environment of the Neogene foreland basin.

Preface: New advances in unconventional petroleum sedimentology in China

Significant progress has been gradually made on the exploration and development of unconventional petroleum on land in China over the past 20 years. In 2019， unconventional petroleum production in China accounted for about 23% of the total oil and gas production that year and reached 70 million tons of oil equivalent in 2020, indicating China’s petroleum industry has entered a new stage of unconventional petroleum development. Unconventional petroleum sedimentology as an important part of unconventional petroleum geology， and the theoretical system has been gained more and more attention over recent years， leading to important studies， such as “Depositional model of gravity flows， sandy debris in a deep lacustrine basin”， “Depositional model of marine and/or lacustrine organic-rich shale”， “Abundant micro- and nano- pore throat systems developed in fine-grained sedimentary rocks”， and “Sweet-spot areas （intervals） of unconventional petroleum resulted from coupling the sedimentology of several geological events”. This special issue systematically assembles new advances made in the unconventional petroleum sedimentology of China by Chinese unconventional petroleum sedimentologists in recent years. These studies refer to about 30 sets of unconventional petroleum strata （intervals） from the Proterozoic to Cenozoic in more than 50 basins， providing an important theoretical basis and technical support for unconventional petroleum exploration and development. Unconventional petroleum sedimentology would focus on deposition and formation of“sweet⁃spot box” and “sweet⁃spot group ”developed in unconventional petroleum industry in future, and further enhance efficiency of unconventional petroleum exploration and development.

Role of sea-level change in deep water deposition along a carbonate shelf margin, Early and Middle Permian, Delaware Basin: implications for reservoir characterization

Abstract The architecture and sedimentary characteristics of deep water deposition can reflect influences of sea-level change on depositional processes on the shelf edge, slope, and basin floor. Outcrops of the northern slope and basin floor of the Delaware Basin in west Texas are progressively exposed due to canyon incision and road cutting. The outcrops in the Delaware Basin were measured to characterize gravity flow deposits in deep water of the basin. Subsurface data from the East Ford and Red Tank fields in the central and northeastern Delaware Basin were used to study reservoir architectures and properties. Depositional models of deep water gravity flows at different stages of sea-level change were constructed on the basis of outcrop and subsurface data. In the falling-stage system tracts, sandy debris with collapses of reef carbonates are deposited on the slope, and high-density turbidites on the slope toe and basin floor. In the low-stand system tracts, deep water fans that consist of mixed sand/mud facies on the basin floor are comprised of high- to low-density turbidites. In the transgression and high-stand system tracts, channel-levee systems and elongate lobes of mud-rich calciturbidite deposits formed as a result of sea level rise and scarcity of sandy sediment supply. For the reservoir architecture, the fan-like debris and high-density turbidites show high net-to-gross ratio of 62 %, which indicates the sandiest reservoirs for hydrocarbon accumulation. Lobe-like deep water fans with net-to-gross ratio of 57 % facilitate the formation of high quality sandy reservoirs. The channel-levee systems with muddy calciturbidites have low net-to-gross ratio of 30 %.

Matrix granulometry of catastrophic debris flows (December 1999) in central coastal Venezuela

After 2 days of intense precipitation, devastating flooding and debris flows affected the central coast of Venezuela on December 15–16, 1999. These events killed nearly 30,000 people and destroyed or damaged 126,000 homes. I visited the disaster area in March 2000; coastal valleys affected by debris flows were extensively covered by an interstitial matrix that included large amounts of often impressive boulders, up to 35–40 tons in weight. Granulometric analyses of four matrix samples from the Naiguatá valley showed they were all similarly coarse; their main constituent was sand, and grains <0.063 mm (silt and clay) made up only 1.3–14.2% by weight of the matrix. Median ( D 50) matrix grain-size was used to calculate—by interpolation on published diagrams (Water Resources Research, 21 (1985) 1511)—the minimum sediment concentration needed for debris flows to reach the yield-strength threshold required to mobilize large blocks. The data imply that debris flows needed at least 53–62% sediment by volume to transport matrix-supported boulders. Given the possibility that some of the fines may have been lost in suspension during flow or by subsequent flooding, I recalculated median matrix grain-sizes assuming that either 50% or 75% of the silt and clay had not been preserved in the deposits sampled. The new D 50 values did not alter results substantially, as a 50–57% sediment concentration would have been still needed for such rough matrix to support boulders. Alternatively, the textures discussed here may approximately describe the actual particle-size distribution of the matrix involved in transporting boulders during the debris flows. This is suggested by the presence, 6 months after the flooding events, of a remnant sandy matrix still preserved beneath and between large boulders (Wieczorek, personal communication, 2000 and 2001. United States Department of the Interior, U.S. Geological Survey, Reston, VA 20192, USA). The above implies that the Venezuelan debris flows were of the granular, noncohesive type (cf. Scott, K.M., Vallance, J.W., Pringle, P.T., 1995. Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington. U.S. Geological Survey Professional Paper 1547, 56 pp.). Data suggest the following events must have taken place in coastal Venezuela. Torrential rains quickly caused flooding along creeks and rivers, which must have rapidly attained hyperconcentrated streamflow. Bulking of stream alluvium in flood surges may also have taken place. However, the storm triggered widespread landsliding along the steep, unstable flanks of the Coastal Cordillera; many slides started in upper-basin areas, underlain by gneiss bedrock (Peña de Mora). Such slides released vast amounts of coarse, sandy debris, which mixed with turbulent floodwaters to produce numerous granular, noncohesive debris flows. These flows, despite their coarse matrix, exhibited intense particle collisions and interactions, thus were able to effectively mobilize vast masses of large boulders.

A Note on the Distribution of Primula Scotia Hook

In the past the distributional pattern of Primula scotica has stimulated a certain amount of speculative discussion of the phytogeographical problem which it presents. The focus of previous examinations of this and other related problems-such as the Teesdale 'refugium'-has been whether the present-day area is the result of per-glacial survival or of post-glacial colonization. Godwin (1949) has initiated a recent reorientation of emphasis by indicating that the increasing mass of evidence that many of our native species have occupied, in Full-glacial and Late-glacial times, areas which were greater than those within which they are extant, implies that the basic problem is 'essentially one of post-glacial movements and adjustments rather than of per-glacial survival'. P. scotica is a member of the Artic-Subarctic element (Matthews, 1937), and the macroscopic remains from the Cam Valley deposit are ascribed by Godwin (1953) to the Fullglacial period. Discussing the factors which might have effected a restriction of the areas of plants of which there is evidence for greater areas in the past, Godwin (1949) suggests as the most important the spread of dense forests and peat mires with a complementary disappearance of open habitats with base-rich mineral soils. Further, he indicates that the first phase in this sequence of substrata was one of 'sheets of dead ice decaying in situ, often covered with gravel and sandy debris and even vegetation clad before their final disappearance'. In the light of this new approach, an examination of the present-day distribution of this plant in relation to its ecological amplitude is of value in a study of the environmental changes in the past which might have effected a reduction in the size of the area occupied by the plant, while it is recognized that a complete basis for accounting for the history of the plant can be established only from fossil records. The significance of seed dispersal and the availability of suitable habitats in determining the distribution is recognized, and will be considered along with the ecological amplitude of the plant.