What are clastic sediments give examples?

2.8.1 Clastic Sediments

Clastic sediments are predominantly clay minerals and quartz particles, with minor amounts of Feldspars, micas, and heavy minerals. Porosity results from the space between the grain particles that is not filled with cement or clay. Porosity is usually in the range from 10% to 30% depending on the grain sizes, compaction, and the amount of cement present between the pores. Permeability, which is the property that permits fluid to flow through the pores, is controlled by the amount of cement, the degree of compaction, and the magnitude and variation of grain sizes.

Sediment production

Clastic sediments are produced by the physical disaggregation of preexisting rocks during weathering and mechanical erosion. Chemical weathering weakens rocks by altering mineral compositions and by removing the minerals cementing them together. Physical weathering involves fragmentation due to tensional or compressive stresses, including expansion of clay minerals within rocks, frost shatter, and root wedging. The latter two are frequently sources of angular rock fragments, including breakdown, in and near cave entrances (Fig. 1). Breakdown blocks are clastic sediment and they easily exceed the total volume of all other clastic deposits in a cave; occurring where passage widths exceed the tensile strength of ceiling or overhanging rock beds. A detailed discussion of breakdown and its formation is provided elsewhere in this volume.

Cave streams mechanically erode conduit walls through a combination of plucking and corrasion. Plucking adds sediment to karst streams when intense hydraulic forces pull or wedge blocks of rock from conduit surfaces. Corrasion occurs when sediment grains carried by floodwaters strike bedrock surfaces with sufficient force to break off fragments. This process is generally referred to as abrasion when the result is a smooth or polished cave surface and percussion when it results in obvious flaking and rock breakage. Percussion generally implies impact of large grains, such as cobbles, whereas abrasion is most commonly associated with “sand blasting” by suspended sediment. As reported by Malcolm Newson, solid carbonate grains created by abrasion can collectively weigh more than the dissolved solids in those same cave-fed floodwaters. Whether corrasion plays a major role in cave development elsewhere is a matter of ongoing research, but it is undoubtedly a nontrivial source of sediment to high-energy cave streams where large grains impact cave walls.

Carbonates may contain chert or other low-solubility minerals, which enter cave passages as the surrounding rock is eroded. Where abundant, chert can be the major or only significant autochthonous bed load supplied to cave streams. In other cases, comparatively high concentration of autochthonous grains may accumulate in clays and silts when sedimentation rates are otherwise very slow (Fig. 2), perhaps recording protracted pipe-full conditions in a quietwater setting (many hundreds of years) (Table 1).

How Deepwater Sediments Are Formed

The sedimentary rock cycle is illustrated in Figs. 2.15 and 2.16.

Clastic sediment is sediment consisting of fragments of rock, transported from elsewhere and redeposited to form another rock. Clasts are individual grains that make up the sediments. The sediment particles are then further exposed to rain, wind, and gravity, which batters and break them apart through further weathering and erosion processes.

The products of weathering will finally include particles ranging from clay to silt, to pebbles and boulders, that are then suspended and transported downstream by wind, streams, rivers, and ocean tides and currents to the earth's ocean and sea basins below, where they are buried, lithified, subjected to heat and pressure at various depths to solidify into the many different sedimentary rock types that exist.

As the earth consists 70% of water, a great majority of sediments will form into the estuaries, deltas, seas, lakes, and oceans to form sedimentary sequences that will often result in kilometers of sedimentary rock sequences below the subsurface, i.e., seabed, where, when deep enough, further pressure, heat, and temperature changes further cook and change the sedimentary rock.

Above the metamorphic bedrocks within the earth basins, sediment thicknesses overlying the majority of the world’s oceans, seas, and margins have been mapped, interpreted, and can be readily obtained to conclude deepwater sedimentary basin sequences and rock thickness where hydrocarbons exist are not all the same.

Ocean sediments are products of weathering, erosion, and transportation through layered streams of sand, silt, mud (clay), and other materials (carbonates) further precipitate from solution. These materials then are deposited on the continental ocean and sea floors as tectonic plates converge, diverge, rise, or subside to form ocean ridges or other unique seabed features to form the world's deepwater sedimentary ocean floors and drilling basins that exist today.

Clastic Varved Lake Sediments

Clastic sediments predominate under cold climatic conditions, such as those found in the Arctic (Figure 3) or in high Alpine regions (Figure 4). Such sediments are typical for proglacial and periglacial lakes. Intensive physical weathering and the lack of a densely vegetated catchment area provide high amounts of minerogenic detritus, which is easily eroded and transported into the lake. The sediment transfer is related to the annual freeze–thaw cycle and the amount of runoff. In regions with a continental climate, runoff is governed by melting of snow and ice through solar insolation during summer. Under oceanic climatic conditions, runoff is controlled either by the melting of snow and ice through advective heat transport or by precipitation. Such lakes are poor in nutrients (oligotrophic), which inhibits high organic productivity and the formation of organic varves.

Clastic varves are formed when a discontinuous (highly seasonal) stream loaded with suspended sediment enters a lake of stratified water (Sturm, 1979). According to its density and in relation to the density of the lake water, the inflowing stream water is positioned in the lake as over-, inter-, or underflow. Over- and interflows cause a wide distribution of suspended matter across the lake. After entering the lake, the flow velocity of the stream water is transformed into turbulence causing a reduction of transport capacity for suspended sediment particles. Accordingly, coarse particles (sand, silt) are deposited immediately, whereas fine particles (fine silt, clay) remain in suspension until runoff has stopped or the lake is ice-covered. Thus, clastic varves are composed of a pale coarse-grained basal lamina and a darker fine-grained (clay) top lamina (Figure 3) or vice versa (Figures 4 and 5; Table 1). Additional coarse-grained laminae are frequently observed and are often related to successive annual runoff events.

Table 1. Different varve types with idealized composition, color, and timing of corresponding seasonal laminae

Alleghanian orogeny

Clastic sediments derived from the interior of the orogen began in the late Mississippian in the southern and central Appalachian foreland, and in basins in the interior of the orogen from New England to Newfoundland (Figure 1). These interior basins are considered stepover rhomb grabens related to large dextral faults (Cobequid–Chedabucto, Bellisle, Cabot), products of the initial stages of collision of Gondwana with Laurentia, and the closing of the Theic ocean, which was followed by the final assembly of Pangea (Bradley, 1982) (Figure 7). Coeval with or slightly younger than deposition, amphibolites-facies metamorphism, polyphase deformation, and plutonism are recorded in southern New England and in an antiformal belt along the Coastal Plain overlap from Virginia to Alabama, variously called the Goochland terrane–Raleigh belt, Kiokee belt, and Pine Mountain terrane (Figure 1). An Alleghanian thermal event is also recorded across much of the Tugaloo and Cat Square terranes in the south (Dennis and Wright, 1997; Merschat et al., 2005), and is much more widespread across southern New England than was previously thought (Wintsch et al., 2003; Walsh et al., 2007).

The basic outline of the southern and central Appalachians is dominated by Alleghanian structures that inherited the Neoproterozoic shape of the continental margin (Thomas, 2006). Foreland deformation is Alleghanian, and timing of faulting in the domain of large strike-slip faults in the interior from the Brevard fault eastward is predominantly Alleghanian, with the exception of the Neoacadian Central Piedmont suture, but it too was locally reactivated during the Alleghanian (Figure 3). Farther north, the Alleghanian record is confined to the interior of the chain, with several plutons and extensive metamorphism in New England (including the large Sebago batholith in Maine) (Wintsch and Sutter, 1986; Walsh et al., 2007), and several major dextral faults on the eastern side from southeastern New England to Nova Scotia, which reappear in interior western Newfoundland (Figure 2). The large, mostly stepover basins containing Carboniferous and Permian sediments are directly related to these faults (Bradley, 1982; Mosher, 1983). LeFort and Van der Voo (1981) and LeFort (1984) suggested that the Reguibat Promontory in West Africa collided with Laurentia here before collision of the main African continent in order to explain the narrow, strongly curved segment of the Pennsylvania salient in the central Appalachians (Figure 7). They concluded that collision of the promontory produced an escape tectonics scenario, where dextral faults facilitated southward escape of crustal blocks and sinistral faults carried blocks northward out of the collision zone. The movement sense of the array of Alleghanian faults south of the projected collision zone, including the Brevard, parts of the central Piedmont suture, and all of the eastern Piedmont fault system (Figure 1), is clearly dextral, but the movement sense of faults north of the collision zone, including the Clinton-Newbury, Bloody Bluff, and all of those farther north, is also dextral (e.g., Bothner and Hussey, 1999; Goldstein and Hepburn, 1999). Based on the ages of stratigraphic sequences and fault kinematics, I have proposed that the collision involved both rotation and transpression, and that collision began at the northeastern end of the Appalachians and closed the Theic ocean southward like closing a zipper. In this scenario, Gondwana would have rotated into head-on collision with southeastern Laurentia in Late Carboniferous to Permian time, producing the Blue Ridge–Piedmont megathrust sheet that pushed foreland deformation in front of it from southern New York to Alabama (e.g., Hatcher, 2002; Hatcher et al., 2007c). The zipper tectonics scenario fits more of the data related to the tectonostratigraphic and kinematic closing of the Theic ocean and final assembly of Pangea than others. The Alleghanian orogeny in Laurentia, and the equivalent Variscan components in Europe (including the Uralian orogeny; Pushkov, 1997), joined all existing continents to create supercontinent Pangea. This process took some 490 m.y. from the time of initial rifting of Rodinia to the final assembly of Pangea. Appalachian history began with the Neoproterozoic breakup of supercontinent Rodinia, and the rifting and progressive separation of southeastern Laurentia from Gondwana, opening the Iapetus ocean. Once rifting began, a continuous record of the transition of rifted margin to stable platform depositional conditions existed on the Laurentian margin. The Appalachian orogen may be unique among orogenic belts in that three major orogenies affected the entire orogen during its development, one of which involved arc accretion (Taconian), while the other two involved large terrane (Acadian–Neoacadian) and continent-continent collision (Alleghanian) that completed the Paleozoic Wilson cycle, forming supercontinent Pangea. These three orogenies produced widespread penetrative deformation, metamorphism to at least amphibolite facies, volcanism, suites of felsic and mafic plutons, and diachronous clastic wedges throughout the orogen (Taconian) or restricted to the southern and central Appalachian foreland (Acadian-Neoacadian and Alleghanian). These clastic wedges clearly track diachronous events taking place deep inside the orogen or along paleo-ocean margins.

Actually, the uniqueness of the Appalachian events is paralleled by similar almost coeval events in the Variscan and Uralian orogens. Similar Ordovician, mid-Paleozoic, and Carboniferous–Permian events occurred in the Variscan of Iberia, in western central Europe, and in the Urals (Pushkov, 1997; Martínez Catalán et al., 2002, 2007; Matte, 2002). Matte (2002) presented an interesting comparison of the plate-tectonic processes that occurred almost at the same time in all of these orogens, illustrating their parallel development.

4.3.1 Carbonate platform

If clastic sediment supply to the shelf is limited and the water temperature is right, then a carbonate dominated shelf develops with high-energy organic build-ups along its edges fringing the deeper basin (Figure 4.135). Carbonate rocks are subdivided into several types based on the presence of lime-mud and their mode of deposition (Dunham's classification, Figure 4.136). The packstone, grainstone, rudstone and boundstone lithologies form the higher energy members, while the mudstone and wackestone are deposited in less agitated water. Micro-graphs illustrate the different textures and carbonate rock types (Figure 4.137). Rud- and boundstone are made up of colony-making organisms like corals, which provide a solid frame skeleton for the rocks (Figure 4.138). Coral growth depends on:

Temperature.

Availability of nutrients.

Proper amount of sunlight (photic zone, water depth).

Salinity.

Clear and oxygenated waters.

Limited input of amount of silici-clastics (Figure 4.139).

Nowadays also deep water reef communities are found below the photic zone till a water depth of 300–500 metres (offshore Hawaii and Florida channel, G. Eberli). Corals are also found in the Norwegian waters, so even temperature conditions for carbonate growth and deposition varies a lot. On the eastern margin of the Rockall Trough (offshore NW Ireland) the cold water carbonate mounds are fed by strong nutrient rich upwelling of bottom currents and/or faultzone related escape of thermogenic or hydrate gasses (O’Reilly et al. 2005). Higher to moderate temperatures in a basin will usually stimulate the biological activity and hence the sedimentary productivity. Lime-mud is also known as micrite, while crystalline carbonate cement is called sparrite (Folk's classification). Mud is usually deposited in a quiet low-energy sedimentary environment.

Carbonate platforms are zones in the offshore, where the biologic productivity is high and only a limited amount of silici-clastics reach the marine offshore shelf (cf Sellwood 1979, Scholle et al. 1978). A typical subdivision on a carbonate platform is illustrated in Figure 4.140. The reef is growing along the shelfedge on the border with the deeper marine offshore environment. Contrary to silici-clastics shelves, the carbonate reefs generate their own sediment supply. The waste material is dumped in front of the reef on the talus or slope. Debris generation and mobilisation occur mainly under the influence of stormy events. Many times a foresetted and progradational geometry in the slope deposits is the result. The reef protects the lagoon behind it, where finer grained material is accumulating and the occasional patch reef can grow (cf Bebout and Louks 1974). Reefal debris is also dumped into the lagoon bordering the reefal complex. Local patch reefs may exist on the carbonate platform itself, when growing conditions for the framework building organism are favourable (e.g. Booler and Tucker 2002).

Large scale foresetting is particular well developed in the Tamabra deposits offshore the Golden Lane platform in Mexico (Figures 4.141 and 4.142). The foresetting of the talus deposits may be used as a diagnostic criterion for the presence of a reefal body in an updip position (Figure 4.143).

A field example of a mini Mesozoic patch reef on a carbonate platform is illustrated in Figure 4.144. An oblique mounded geometry is recognised with some foresetting in the talus apron in front of the high-energy reefal complex. Mesozoic platform carbonates in the Paris basin have a wide spread distribution. These limestone deposits are monotonous, well bedded and often with little thickness variation (Figure 4.145). A small shale break is indicative that the carbonate sedimentation is shortly interrupted and then the same sedimentation pattern continues again. Bioclastic offshore bars in Oxfordian carbonates on the eastern flank of the Paris Basin at Euville (France) have been described by Dagallier et al. (2000). The outcrops illustrate rapid progradation of bioclastic talus sediments, quarried as decorative building material blocks. Their 3D geometry has been studied with the aid of high-resolution georadar data. The detailed 3D structure of these reefal complexes is revealed on high-resolution GPR data. The georadar sections are directly calibrated by the geologic formation seen in the field and the cliff faces in the quarry. The GPR (or Ground Penetrating Radar) method is popular in academic research because it is cost effective and simple to use. The system is highly portable and can even be deployed on a water surface with an inflatable vessel or even under water. It provides an elegant way to define high-resolution 3D reservoir working models, suitable for detailed fluid flow characterisation. Models, based on combined 2D outcrop data alone, are most of the time not sufficient. Proper calibration of spatial relationships is an important matter in geoscience (cf Zeng et al. 2004, Pringle et al. 2004).

The seismic example of a Triassic reef complex illustrates a lenticular mounded shape in cross-section (mounded in 3D). The foresetting and the facies change from talus into the core of the reef are outlined and there is also evidence for a protected lagoon behind the reef complex (Figure 4.146).

Oolites are typical limestone deposits, created in a high-energy environment. An oolite is a well-rounded grain particle, formed by the concentric precipitation of calcite around a nucleus. It is the wave action that rolls the nucleus particles back- and forwards creating the symmetrical aspect of the oolite. The oolite deposits can build massive shoals and offshore bars with foresetting on the shelf (Figure 4.147). It constitutes a chemical precipitation around a moving grain. Oncolites are the product of algal coating around a particle or grain. Their shape is much more irregular and reflects a lower energy environment. Oncolites are slightly bigger in size (1 - 2 cm) than the oolites. The high-energy oolite deposits show excellent reservoir characteristics because of their sorting and the rounded nature of the grains. Bi-modal sorting may result from selective biologic activity, whereby sediment taken up by the organism passes through the digestary intestine tract producing a coprolite or burrow with special characteristics. This burrow sediment is reworked again giving rise to a bimodal grain distribution.

Fine-grained sediments accumulate in the protected lagoon, where the wave energy is reduced. They are characterised by a parallel to wavy bedding with fine internal laminations. These laminations or stromatolites are mostly stemming from algal mats. Local small-scale undulations are quite frequent and underline a wavy aspect. Coarser material in the lagoon are wash-over fans, created during extreme storm events. But also a bayhead delta may exist where silici-clastic material is brought into the basin. A tidal inlet and tidal delta may give connection to the open marine deeper waters at discrete entry points of the lagoon. Sometimes the relation between carbonate and clastic deposition is quite intricate. If the conditions are right (temperature, oxygenate water, enough nutrients, sunlight) then a bioherm can grow and carbonate deposition will locally take place, even in an overall clastic environment (Figure 4. 148).

On seismic sections the reefs may have different characteristics. A mounded shape is often apparent, associated with shingling low-angle foresetting in the talus deposits (Figure 4.149). Time slices and a coherency cube illustrate the different facies units present in a reef complex (Figure 4.150). The carbonates have a higher interval velocity and below the reef body a pull-up velocity effect on seismic twoway time sections can be expected. The high-energy facies can be transparent or chaotic in reflection character. Reefal build-ups (mounded, foresetted) protect the inner shelf, where lagoonal/inner shelf carbonates and shales are deposited. Thes deposits may have their own seismic facies expression that stands out from their surroundings. Careful amplitude evaluations may reveal their presence (Figure 4.151).

The carbonate platform is often fringed by a high-energy reefal barrier complex. Local patch reefs may grow on the protected carbonate platform. The platforms are expressed on seismic datasets by mostly sub-parallel, variable amplitude facies units, with high continuity reflections. The wide spread nature of carbonate platforms is demonstrated by the Cretaceous Chalk deposits in the western Europe. The relief in the hinterland was rather low and a limited amount of clastics was transported to the marine basin. The climate was hot and the sea covered large areas during the Upper Cretaceous sealevel highstand. The reefal build-ups may look on the seismic sections as reflection free zones with on their seaward side indications of foresetting, pointing to larger water depths. The shape of the build-ups is prograding, aggrading or retrograding depending on the degree of relative sealevel rise.

Under relative sealevel rise conditions the carbonate shelf edge can be rather steep. This geometry is maintained due to little erosion on the platform and hence relatively low sedimentation rates on the basinfloor. The shape of the slope depends a lot on the amount of material available for re-deposition at the slope and base-of-slope regions. Storm-generated wave energy is important as an eroding agent and transport of shelf sediments (cf Veeken et al. 1999). In addition, tectonic slope instability triggers large quantities of massflow currents. As said already, prograding slope systems with submarine fan complexes at their base are recognised within carbonate rocks. Cretaceous limestones in the southern Pyrenees are good examples of this (Drzewiecki and Simo 2002). Reefs may constitute prolific hydrocarbon reservoirs with good porosity and permeability (e.g. Arthur and Schlanger 1979).

If the relative sealevel fall is large enough, then the reefs and bordering shelf area are submitted to subaerial erosion. Under such circumstances the carbonates are leached and the porosity of the rocks is increased (e.g. Jurassic Casablanca field, offshore Spain; Watson 1982). Karst is an extreme weathering type found in exposed carbonate regions. It is characterised by extensive subterraneous cavern systems. High losses of drilling mud, resulting in severe problems in pressure control for a well, are some of the dangers associated with these zones (drilling hazard).

Dolomitisation, resulting in a change in porosity / permeability, takes place when the carbonates are invaded by pore fluids rich in magnesium. There exist three basic mechanisms for producing dolomite in carbonate rocks:

Evaporation related dolomitisation, interlayered with carbonates, parallel to the bedding.

Mixed water related dolomitisation, cross-cutting the original layering. It is fact an early diagenetic process.

Burial related dolomitisation, also cross-cutting the layering. Updip migration of expelled magnesia-rich brines from compacting shales.

Dolomitisation often increases the permeability of rocks and therefore improves the overall reservoir quality (e.g. Mancini et al. 2004). Anhydrite cement is formed as the initial transformation product in virgin limestones invaded by the reflux groundwater in the dolomitisation process. The magnesium contents of the groundwater is enriched by evaporation.

Carbonate rocks may form good reservoir for trapping hydrocarbons (Moore 2001). The coarser deposits have more chance to retain good porosity and permeability. These are normally found in the higher energy sediments (reef and reef talus). Leached deposits with secondary porosity are of special interest. Moldic vuggy porosity is not connected and therefore not good for the permeability. The shape of the hydrocarbon trap in limestone rocks may be very irregular and controlled by the diagenesis. Carbonate cementation can start very early in the burial history, contrary to silica cement that precipitates only above 70 degrees Celcius.

The source rock potential of the Mumbai offshore basin

The clastic sediments in the lower Eocene to Paleocene sedimentary sequences (Panna Formation) are the principal source rocks across the basin. The thickness of the source rock varies from 30 to 1000 m depending on location. The excellent source rocks of restricted marine to lagoonal deposits within the Panna Formation in the Central graben and adjoining area are the prime source of hydrocarbon accumulation in the basin. In the Mahim graben, a 400-m-thick sequence in the Panna Formation contains very good/excellent oil-prone effective source-rock facies, which account for the commercial petroleum reservoirs within Bassein, Mukta and Heera formations in the east of Panna and Bassein fields (average TOC = 2.3–15.4%; average S2 = 3.5–50.1 mg HC/g rock; average HI = 112–277 mg HC/g TOC). Organic-rich mature source-rock sequences in the Panna Formation occur in depressions across the DCS area and west-southwest of Mumbai high (average TOC = 1.5%–5.6%; average S2 = 2.6–11.6 mg HC/g rock; average HI = 94–270 mg HC/g TOC). Source-rock data from the deepest exploratory well in the Vijaydurg graben of Ratna depression show good, mature source-rock section in the lowermost unit of the Panna Formation and thin coal and coaly shale layers with very good source-rock quality at the top of Panna Formation. The sedimentary column in Shelf Margin areas is dominated by clastics, except in the middle Eocene, which has carbonates. The source-rock potential of the Paleogene sediments is moderate, but some good organic carbon-rich source-rock layers are present in Neogene sediments (Goswami et al., 2007).

Paleocene-Early Eocene sequence is characterized by the richest concentration of organic carbon (TOC) within the Tertiary succession of the Mumbai basins. The Hydrogen Index (HI) order of 150–250 in the Heera-Bassein of the northern Ratnagiri areas indicates high TOC. Good concentration of TOC has been observed in the sediments of the Middle to late Eocene sequence in the Surat depression and in a narrow linear area in the western part of the Mumbai High platform. This sequence is matured adequately and the Time–Temperature Index (TTI) values are varying between 10 and 100. The sediments of Oligocene sequence in the Murud depression shows good to moderate TOC concentration. Studies indicate that the organic matter in this sequence is of Type III and the sediments are in the early stage of oil generation. The sediments of the Early to Middle Miocene sequence contain a good concentration of TOC, which is more than 1.5% in the Kori-Comorin depression. There is a moderate concentration of TOC in the central part of the Surat depression. The sediments in these areas are in the early stage of oil generation.

7.5 Summary

Paleogene lacustrine clastic sediments are known to be the main source rocks in the Cenozoic rift basins of Southeast Asia (Longman, 1993) and China (Watson, Hayward, Parkirkson, & Zhang, 1987). Paleogene is the period of strong regional extension and rapid initial subsidence in most rifting basins in the China Seas. Deepwater environments were favorable for deposition of organism-rich mudstones. Eocene lacustrine carbonate source rocks and reservoirs have been found in the Bohai Basin (Peng, 2011; Wang, Wang, & Zhang, 2010). Another secondary period of source rock deposition is the early Neogene, when the thermal subsidence just started with relatively fast subsidence rates.

Source rocks of marine facies are also important in the SCS and southern part of the ECS. The SCS area had two major episodes of marine deposition, in late Paleocene/early Eocene and late Oligocene–early Miocene, respectively (e.g., Jin, 1989; Shi & Li, 2012). Therefore, most basins around the SCS have both lacustrine and marine source rocks. While most source rocks were deposited during the early rifting phase of basin development before Miocene, Miocene source rocks exist in basins/depressions that are with higher geothermal gradients and normally close to the central oceanic basin of the SCS. Paleogene source rocks became mature in the middle to late Miocene, slightly later than the formation of most traps (Jin, 1989).

Reef reservoirs become extremely important in the SCS. Late Oligocene and early Miocene marine carbonate platforms and coral reefs developed in the early spreading of the SCS are primary reservoirs. In the Miocene, they occurred all around the SCS and often developed on structural highs between early rifted half-grabens (e.g., Williams, 1997). Hydrocarbons generated within the grabens can migrate upward into these carbonate structures.

Tectonics exerts critical roles during the entire life span of a petroleum system. Paleogene tectonics controls not only the basin configurations and distributions of source/reservoir rocks but also the migration and accumulation of hydrocarbons. Synrift faults can be important pathways of hydrocarbon migration and can form fault-related traps. Late Miocene and Quaternary tectonic activity offshore China also generated unconformities and new faults and formed new structural traps, migration routes, and source kitchens (Gong et al., 2011). Neotectonism can modify preexisting hydrocarbon accumulations and formed new large pools. For example, the strong neotectonism associated with large strike-slip faults in the Bohai Basin and Yinggehai basin is found to be closely related to large oil and gas pools there (Gong et al., 2011).

Large biogenic gas fields discovered so far in China are mainly distributed in the continental shelf basins—the Yinggehai–Qiongdongnan basin, PRMB, ECS basin (Chunxiao gas field), and Taixi Basin (Figure 7.1). Carbon isotope of ethane from these fields is heavier than − 28‰, indicating that the gas is coal-derived (Dai et al., 2009). Thermogenic gases are also dominant in the Yinggehai and Qiongdongnan basins, where the geothermal gradients are high (Huang et al., 2003).

Mineral resources in the China Seas include coastal placers, phosphorite, sulfide deposits, and ferromanganese nodules/crusts. Recently, many research activities were carried out on gas hydrates, which are found in the SCS and Okinawa Trough.