Shunli Li

Classification, formation, and transport mechanisms of mud clasts

ABSTRACT Mud clasts are common in non-marine to marine sedimentary records, however, why lack a widely accepted classification scheme? We propose that it is the relative balance of volumetric abundance, sorting, roundness, and grain size that controls the texture and fabric of mud clasts. Nine distinct types of mud clasts are identified in the study based on quantitatified properties, and fall into two groups coarse-grained and fine-grained. The generation of mud clasts can be assigned to failure, erosion, and/or bioturbation of muddy sediment. These clasts are transported within fluid flows including Newtonian fluids, non-Newtonian fluids, and Bingham plastics (gravity flow and turbidity flow), showing various physical characteristics depended upon the density and viscosity of flows. Newtonian flows with less density and viscosity commonly form mud clasts with mature textures. In non-Newtonian (gravity-driven) flows, mud clasts are normally transported in laminar flows with high density and viscosity, developing matrix-supported mud clasts with immature textures. The study of classification, formation, and transport mechanisms of mud clasts has implications for identifying and interpreting sedimentary environments.

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 %.

Research Methods of Sedimentary Facies and Sedimentation

The identification of sedimentary environments and sedimentary facies is based mainly on various facies markers that are determined and obtained from geology, logging, and seismic data. However, data analysis and research cannot be separated from discussions on the formation mechanism of these markers or sedimentation.

Theory and Methods for Studying Clastic Sequence Stratigraphy

Sequence stratigraphy is the scientific study of rock relationships using a repetitive and genetically geochronologic sequence stratigraphic framework, which is demarcated by an erosion surface, a non-sedimentation surface, or a correlative conformity surface. It is a new approach for dividing, contrasting, and analyzing sedimentary strata. When combined with biostratigraphy and tectonic subsidence analysis, it enhances the feasibility of chronostratigraphic correlation, paleogeographic reconstruction, and prediction of petroleum reservoirs, source rock, and seal bed before drilling. The application of sequence stratigraphy to sedimentary strata can potentially provide a complete and unified stratigraphic concept, similar to the complete and unified structure concept for plate tectonics. Moreover, sequence stratigraphy has changed the fundamental principle for stratigraphic record analysis and started a new stage in understanding the history of the Earth. As a result, it triggered a revolution in geology in the 1980s.

Fluvial Depositional System

Rivers are not only the main geological agent for corroding and transforming continental topography and carrying weathered materials to lakes or seas but are also important sedimentary agents in continental areas. Given an appropriate structural environment and sedimentary background, sometimes a fluvial sedimentary stratum with thickness of thousands of meters can develop.

Basic Features of Clastic Reservoirs

Oil and gas reservoirs worldwide are dominated by sedimentogenic clastic rock and carbonate rock strata. Therefore, it is necessary to study the relationships among the various sedimentary environments of petroleum reservoirs, paleogeographic conditions, spatial distribution features of sedimentary systems, and various sedimentary facies belts to build a reservoir depositional model and geological model.

Lacustrine Depositional System

Lakes are low-lying areas that collect running water on land. Currently, the total area of lakes all over the world is approximately 250 × 104 km2, which accounts for 1.8% of the global land area. In China, only 1% of the land area is covered by lake water. Lakes were scattered like stars in China during the Meso-Cenozoic era. More than 10 large oilfields, such as Daqing, Shengli, Liaohe, and Dagang, were found in these lacustrine strata. They constitute a feature of Chinese petroleum geology and occupy a unique place in the global petroleum industry.

Sandy Coast (Shore) and Neritic Depositional System

As we know, the marine environment is quite different from the continent. In addition to physical and chemical conditions, there are great disparities between coastal and submarine deposition conditions and processes. Because submarine environment characteristics are related to seawater depth, the sea area can be divided by seawater depth into four zones, shore (coast), shallow sea, semi-deep sea, and deep sea. The continental shelf (also called shelf), continental slope, continental rise (also called continental apron), and ocean floor (also called abyssal plain) can be divided by the geomorphic features of the continental margin. Shelf break is the turning point (or zone) of the continental shelf and continental slope.

Deep-Water Depositional System

As global petroleum exploration constantly goes deeper underground, exploration based on architecture has matured, and all architectures that can be found have been explored in petroliferous basins with relatively high degrees of exploration. Thus, in the future, exploration should focus on looking for lithostratigraphic reservoirs. The presence of a bathymetric or lowstand fan is the main condition for the formation of stratigraphic traps. As a result, studying deep-water depositional systems, particularly deep-water gravity flow depositional systems, is very important. One of the main scientific challenges in the 21st century is understanding resource formation and exploration mechanisms in deep water (sea).

Deltaic Depositional System

As early as 1885–1890, G. K. Gilbert studied the Pleistocene lacustrine delta deposits in Lake Bonneville, America, and first found that the delta deposit body has a three-tier structure. Later, according to Gilbert’s description, J. Barrell (1992 and 1914) studied the characteristics of the sedimentary facies of the Devonian Catskill Delta in the Appalachian Basin, identified the topset, foreset, and bottom set, and respectively depicted the characteristics of the lithology, beddings, and fossils of all formations thus pioneering the study of the sedimentary facies of ancient marine deltas. Although Barrell noted that not all delta deposits have the three-tier architecture of a Gilbert-type delta, the delta depositional models established by Gilbert and Barrell have influenced researchers’ awareness about deltas in the 20th century. Previously, researchers used to consider a megascopic foreset as an important sign for identifying an ancient delta. Moreover, because abundant energy source deposits (including coal, petroleum, and natural gas) having large economic value had not yet been found in delta sedimentary formations, studies on deltas were lacking. As a result, we barely understand the complex and changeable sedimentary characteristics of a deltaic depositional system. Nonetheless, these early research achievements have been of great significance.