Overpressure System Research Articles

Numerical simulation of extensional fault reactivation and fluid flow: Generic models related to the Timor Sea

Structural analyses and interpretation of seismic data from the Timor Sea indicate that most of the faults in the region have been reactivated during the Late Tertiary extensional event. These reactivated faults and the associated fluid flow represent a major risk to trap integrity and hydrocarbon preservation. To explore the controls of extensional fault reactivation on fluid flow in petroleum basins, this numerical modelling study has investigated a group of 3D coupled mechanical deformation and fluid flow models characterized by various simple fault geometries and reservoir stratigraphic architecture. The models simulate a regional-scale block (50×50 km in planview and 10 km in depth) with a larger fault and a small fault (either parallel or cross-cutting). The stratigraphic sequences defined for these models include four rock units. They are, from top to bottom, a carbonate layer, a thin shale layer, a sandstone unit and a relatively soft basal rock unit providing a more ductile domain for fault movement and for reducing the edge effects at the bottom model boundary. Such fault and stratigraphic architecture is a simplified representation of regional structures and stratigraphy in the Timor Sea. Model mechanical and fluid flow properties are based on rock property data for the region, in combination with the general rock property data from literature. To consider rock permeability enhancement with faulting, we have incorporated a simple scheme that allows fault permeabilities to increase with accumulation of shear strain, from their initial values (identical to those of host rocks) to various maximum values depending upon host rock type. Greater shearinduced permeability increase is allowed for the shale unit. Initial fluid pore pressures are important boundary conditions for fluid flow modelling. The current models have investigated the effects of four different initial pore pressure (or hydraulic head) distributions, based on data from the Timor Sea: 1) a slightly over-pressured system in the rocks below the shale; 2) a higher over-pressured system in the rocks below the shale; 3) a slightly over-pressured system but with a small hydraulic head difference between the left and right hand boundaries

Hierarchies of overpressure retardation of organic matter maturation: Case studies from petroleum basins in China

The effects of overpressure on different aspects of organic matter maturation have been examined in three basins using multiple parameters. Overpressure in the Yinggehai Basin has retarded kerogen maturation, hydrocarbon generation, and thermal cracking of long-chain normal hydrocarbons, as well as evolution of isoprenoid hydrocarbons. Overpressure in the Dongpu depression, Bohai Bay Basin, seems to have retarded the maturation of bulk kerogens reflected by Rock-Eval Tmax, the thermal cracking of long-chain normal hydrocarbons, and the evolution of isoprenoid hydrocarbons, but seems to have had no detectable effects on vitrinite reflectance. Overpressure in the Qiongdongnan Basin has had no detectable retardation effects on all aspects of organic matter maturation. The observed phenomena suggest differential retardation of organic matter maturation (that is, different organic matter maturation reactions and different maturity parameters have been retarded by overpressure to different degrees in the same overpressured system) and four hierarchies of overpressure retardation. Hierarchy I retardation is defined as the state where the overpressure has retarded all aspects of organic matter maturation and is proposed to have been caused by the combination of high formation pressure and excess pressure, a high-water/organic matter ratio, and strong retention of reaction products in an early-developed, strong, persistent overpressured system. Hierarchy II retardation refers to the case in which all aspects of organic matter maturation except the maturation of hydrogen-poor kerogens, especially vitrinite, have been retarded and seem to have been caused by early-developed, moderate to strong, persistent overpressure. Hierarchy III retardation refers to the situation where overpressure has retarded only the thermal cracking of liquid hydrocarbons, but has had no detectable effect on the maturation of both hydrogen-rich and hydrogen-poor kerogens, and is proposed to have been caused by moderate to strong, persistent overpressures that began to develop around peak oil generation. Hierarchy IV retardation is defined as the situation where overpressure has had no detectable retardation on all aspects of organic matter maturation and may have been caused by cases wherein the overpressure developed too late, the intensity of the overpressure was too low, the duration of the overpressure was too short, or overpressured fluids had been expelled frequently. Resulting from both the diversities of organic matter maturation reactions and the complexities of overpressure generation and development, the hierarchies of overpressure retardation in different basins may be quite different, and it is necessary to use multiple parameters to understand the evolution of the petroleum systems in overpressured basins.

Tunnel ventilation and safety in escape routes

This paper addresses ventilation principles to provide safe escape circumstances in escape corridors parallel to the tunnel tube and at the tunnel portal. The pressure differences caused by the longitudinal tunnel ventilation system in the tunnel tube and the overpressure system in the escape corridor are discussed. The exit situation of the escape route, near the exit portals, is discussed and proposals to increase the safety are given. The results indicate that with minor modifications, the safety during a fire situation can be increased. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.

Natural gas sources, migration and accumulation in the shallow water area of the Panyu lower uplift: An insight into the deep water prospects of the Pearl River Mouth Basin, South China Sea

Although no drilling has been carried out in the deep water area of the Baiyun sag in the Pearl River Mouth Basin, South China Sea, the successful exploration for natural gas in the shallow water area of the Panyu lower uplift allows an insight into the prospectivity of the adjacent deep water fan system of the Baiyun sag. The Paleogene gas kitchen in the Baiyun sag provided both oil-derived gas and coal-derived gas. Fluid inclusion measurements and 2D numeric modelling of formation pressure indicate four episodes of hydrocarbon charge since the third release of the overpressure system. Seismic wipeout zones manifest several types of gas chimney which could play a role in vertical migration conduits to feed the natural gases into the deep water fan system. There would be, therefore, a low risk for hydrocarbon exploration in the deep water fan system.

Geochemistry and episodic accumulation of natural gases from the Ledong gas field in the Yinggehai Basin, offshore South China Sea

The Ledong gas field, consisting of three gas pools in a shale diapir structure zone, is the largest gas discovery in the Yinggehai Basin. The gases produced from the Pliocene and Quaternary marine sandstone reservoirs show a considerable variation in chemical composition, with 5.4–88% CH 4, 0–93% CO 2, and 1–23.7% N 2. The CO 2-enriched gases often display heavier methane δ 13C values than those with low CO 2 contents. The δ 15N values of the gases range from −8 to −2‰, and the N 2 content correlates negatively with the CO 2 content. The high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the Lower–Middle Miocene source rocks and the formation of abundant CO 2 from the Tertiary calcareous-shales and pre-Tertiary carbonates. The compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the LD221 gas field is attributed to three phases of gas migration: initially biogenic gas, followed by thermogenic hydrocarbon gas, and then CO 2-rich gas. The filling processes occurred within a short period approximately from 1.2 to 0.1 Ma based on the results of the kinetics modeling. Geophysical and geochemical data show that the diapiric faults that cut through Miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs, and that the deep overpressure is the main driving force for vertical and lateral migration of the gases. This gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases, and is also a favorable place for gas accumulation. The proposed multiple sources and multiple phases of gas migration and accumulation model for the Ledong gas field potentially provide useful information for the future exploration efforts in this area.

The relation between cavitation and platelet aggregation during exposure to high-intensity focused ultrasound

Our previous study showed that high-intensity focused ultrasound (HIFU) is capable of producing “primary acoustic hemostasis” in the form of ultrasound (US)-induced platelet activation, aggregation and adhesion to a collagen-coated surface. In the current study, 1.1 MHz continuous-wave HIFU was used to investigate the role of cavitation as a mechanism for platelet aggregation in samples of platelet-rich plasma. A 5 MHz passive cavitation detector was used to monitor cavitation activity and laser aggregometry was used to measure platelet aggregation. Using spatial average intensities from 0 to 3350 W/cm 2, the effects of HIFU-induced cavitation on platelet aggregation were investigated by enhancing cavitation activity through use of US contrast agents and by limiting cavitation activity through use of an overpressure system. Our results show that increased cavitation activity lowers the intensity threshold to produce platelet aggregation and decreased cavitation activity in the overpressure system raises the intensity threshold for platelet aggregation. (E-mail: poliachi@u.washington.edu)

Seismic evidence for fluid migration pathways from an overpressured system in the South China Sea

AbstractOverpressured systems and intense, anomalously hot fluid expulsion in the Yinggehai Basin of the South China Sea offer an opportunity to understand the history of fluid flow and the process of hydrocarbon accumulation in overpressured environments. Fluid migration pathways from overpressured compartments in the basin are largely controlled by the distribution of faults and fractures. Episodic opening of these faults are related to the dynamics of an overpressured system and tectonic movements during basin evolution. At the crests of diapiric structures, fluid expulsion is seismically imaged as chimney‐ or plume‐like features, low to middle seismic amplitudes, and intermittently chaotic and blank reflecting seismic facies. These fluid pathways are controlled by vertical faults, which commonly penetrate overpressured and overlying normally pressured zones. Fluid expulsion is also observed near the main faults, such as the No. 1 Fault at the north‐eastern margin of the basin. Investigation by sidescan sonar on onshore and offshore Hainan Island indicates that there are more than 100 gas seepages adjacent to the No. 1 Fault. Migration pathways in the diapiric structures are controlled by three types of fault and fracture. Penetrative faults formed by dextral strike‐slip movement of the Red River faults commonly occur in the centre of the diapirs, and may have been a triggering factor for the diapirism, and controlled their distribution. Hydrofractures occur in certain mud‐rich layers and may have been generated by hydraulic fracturing. Radial normal faults occur at the top of diapirs and were formed by the intrusive process. These fluid migration pathways played an important role in regional hydrocarbon accumulation.

Geochemistry and origins of natural gases in the Yinggehai and Qiongdongnan basins, offshore South China Sea

Four gas fields and a number of gas-bearing structures have been discovered in the Yinggehai and Qiongdongnan basins offshore South China Sea. Chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins: biogenic gas, thermogenic gas, and gases with mixed origin. Thermogenic gases produced from the Yacheng field are characterized by relatively high contents of benzene and toluene, relatively low δ 13C values of toluene, and high abundance of bicadinanes and oleanane in the associated condensates, showing good correlation with the coal-bearing sequence of the Yacheng Formation in the Qiongdongnan Basin. In contrast, the gases from the DF1-1 and LD gas fields contain high amounts of N 2 and CO 2, low to moderate amounts of benzene and toluene, with relatively high δ 13C values of toluene. These characteristics correlate well with the Miocene neritic shales in the Yinggehai Basin. Analyses on potential source rock samples indicate excellent gas source potential for both the coal-bearing sequence in the Yacheng Formation and the Miocene neritic shales containing type III∼II2 kerogens. As the result of recent rapid subsidence and sedimentation, high temperature and overpressure systems are well developed in the Yinggehai and Qiongdongnan basins. The rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata, whereas the strongly overpressured systems led to retarded organic matter maturation, postponing the time when the source rocks reached peak gas generation. Results of 1-D modeling indicate that the coal-bearing Yacheng Formation in the Yacheng Sag reached peak gas generation during mid Miocene-Pliocene time, after the deposition of upper Oligocene to lower Miocene reservoir rocks. The lower-mid Miocene marine shales reached their peak gas generation stage during the Pliocene-Quaternary in the central Yinggehai basin, supplying abundant gases to charge the Pliocene reservoirs. More importantly, focused-episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks, and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time. The gas–source relationship for the Yacheng field suggests dominantly short–distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the Qiongdongnan Basin.

Characteristics of Overpressure Systems and Their Significance in Hydrocarbon Accumulation in the Yinggehai and Qiongdongnan Basins, China

Abstract Overpressure systems are widely developed in the central depression and palco‐uplift in the Yinggehai and Qiongdongnan basins. They can be divided into three types according to the origin of abnormally high formation pressure in the reservoirs, i.e. the autochthonous, vertically‐transmitted and laterally‐transmitted types. The autochthonous overpressure system results from rapid disequilibrium sediment loading and compaction. In the allochthonous overpressure system, the increase of fluid pressure in sandstone originates from the invasion of overpressured fluid flowing vertically or laterally through the conduit units. The autochthonous overpressure system occurs in the deep‐lying strata of Neogene age in the central depression of the Yinggehai and Qiongdongnan basins. The vertically transmitted overpressure system is developed in the shallow strata of Late Miocene and Pliocene ages in the diapiric zone of the central Yinggehai basin, and the laterally transmitted overpressure system occurs in the Oligocene strata of paleo‐uplifts, such as the structure of Ya‐211 in the Qiongdongnan basin. The results indicate that the autochthonous overpressure system is generally a closed one, which is unfavorable for the migration and accumulation of hydrocarbons. In the allochthonous overpressure system, hydrocarbon accumulation depends on the relationship between the formation of overpressure systems and the spatial location and duration of hydrocarbon migration. The interval overlying the overpressure system is usually a favorable hydrocarbon accumulation zone if the duration of fluid expulsion coincides with that of hydrocarbon accumulation.

Mechanisms of abnormal overpressure generation in Kuqa foreland thrust belt and their impacts on oil and gas reservoir formation

Based on overview for mechanism of abnormal overpressure generation in sedimentary basins, an insight discussion is made by the authors for the distribution, features and generation mechanisms of abnormal overpressure in the Kuqa foreland thrust belt. The abnormal overpressure in the Kelasu structure zone west to the Kuqa foreland thrust belt was primarily distributed in Eogene to lower Cretaceous formations; structural compression and structural emplacement as well as the containment of Eogene gypssalt formation constituted the main mechanisms for the generation of abnormal overpressure. The abnormal overpressure zone in the eastern Yiqikelike structure zone was distributed primarily in lower Jurassic Ahe Group, resulting from hydrocarbon generation as well as structural stress other than from undercompaction. Various distributions and generating mechanisms have different impacts upon the formation of oil and gas reservoirs. K-E reservoir in the Kelasu zone is an allochthonous abnormal overpressure system. One of the conditions for reservoir accumulation is the migration of hydrocarbon (T-J hydrocarbon source rock) along the fault up to K-E reservoir and accumulated into reservoir. And this migration process was controlled by the abnormal overpressure system in K-E reservoir. The confined abnormal overpressure system in the Yiqikelike structure zone constituted the main cause for the poor developing of dissolved porosity in T-J reservoir, resulting in poor physical property of reservoir. The poor physical property of T-J reservoir of Yinan 2 structure was the main cause for the absence of oil accumulation, but the presence of natural gas reservoir in the structure.

Overpressure processing of Ag-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x tapes

Heat treatments of monocore and multifilament Ag-sheathed 2223 tape were carried out using overpressure (OP) processing in a static and a flow OP system between 125 and 180 bar. Mass density measurements and microstructural observations of the oxide core show that OP processing densified the core, achieving up to 95±1% of the theoretical density. Our first runs in the flow OP system yielded J c as high as 41 kA/cm 2 (0 T, 77 K), which is 89% of J c in a fully processed tape with an optimized conventional heat treatment.

Role of high intensity focused ultrasound induced cavitation on platelet aggregation

Our previous studies showed that high-intensity focused ultrasound (HIFU) is capable of producing ‘‘primary acoustic hemostasis’’ in the form of platelet activation, aggregation, and adhesion to a collagen-coated surface. In current studies, 1.1 MHz CW HIFU was used to investigate the role of cavitation as a mechanism for platelet aggregation in samples of platelet rich plasma. A 5 MHz passive cavitation detector was used to monitor cavitation activity, and laser aggregometry was used to measure platelet aggregation. Using spatial average intensities from 0 to 4000 W/cm2, the effects of HIFU induced cavitation on platelet aggregation were investigated by enhancing cavitation activity through use of ultrasound contrast agents, and by limiting cavitation activity through use of an overpressure system. Our results show that increased cavitation activity lowers the intensity threshold to produce platelet aggregation, and decreased cavitation activity in the overpressure system raises the intensity threshold for platelet aggregation. [Research supported by DARPA.]

Characteristics and origin of underpressure system in the Shiwu Fault Depression, south-east Songliao Basin, China

The vertical characteristics of underpressure, pore fluids and sealing condition in the Shiwu Fault Depression, south-east Songliao Basin, are investigated. Based on the pore pressure data from drill stem tests collected from 40 wells, the vertical distribution of pore pressure in this depression consists of a shallow hydrostatic pressure system and a deep underpressure system. The observed low pressures range from about 4 to 10 MPa in the depth interval of about 1550–2800 m. The pore water chemistry data document that the ionic evaporite trends of the pore water in the underpressure zone are different from overlying sediments in the hydrostatically pressured section, indicating that the underpressure system is sealed. The study indicates that the depression has undergone rapid deposition in a rift period during the Cretaceous followed by a long-term slow uplift and erosion in the Tertiary. On the basis of the modelled results of the abnormal pressure evolution in a typical cross-section using a two-dimensional numerical flow model, we believe that the predecessor of the underpressure system was an overpressure system. Since the end of the Cretaceous, the observed underpressure system has developed as a result of a geotemperature decrease ranging from 30 to 70 °C, owing to palaeoheat flow reduction and long-term uplift and erosional cooling. The mudstones below a depth of 1550 m in the deep subnormal pressure system have small measured porosities ranging from 4% to 1.2% with calculated permeability of about 10−21–10−24 m2. The observed underpressure can be modelled if we assign permeabilities below 10−20 m2 with a linear reduction of geotemperature. The geotemperature decrease, in combination with very low permeability in the Lower Cretaceous mudstones, is therefore a possible origin for the formation of the underpressure system in this depression.

Significance of fibrous mineral veins in hydrocarbon migration: fluid inclusion studies

Hydrocarbon migration pathways through impermeable mudrocks commonly involve fracture systems that include fibrous mineral growths, particularly fibrous carbonates and sulphates. The fibrous growths are believed to reflect precipitation in over-pressured systems where fracture-walls are held apart by fluid pressure. Paragenetic studies for such fracture systems often show that hydrocarbon fluids predated the migration of aqueous fluids. Hydrocarbon migration is evinced by traces of solid bitumen along fracture walls or in between individual mineral fibres, and by hydrocarbon fluid inclusions within the mineral in-fills. Mineral in-fills with both hydrocarbon and aqueous inclusions offer the chance to determine palaeo-pressures and confirm if fibrous growths do represent over-pressuring.

Bi-directional fast flow sensor with a large dynamic range

In this article an extended mass-flow sensor is presented. Apart from the magnitude of the flow, as an add-on to the traditional anemometer, this sensor also measures the direction of the flow. This is of interest for the flow sensor market in general, and more in particular for the safety monitoring of low over-pressure systems like surgeries and cleanrooms where the risk of reverse flow needs to be avoided at all times. It detects the smallest amount of gas-flow, down to approximately 100 µm s-1 airflow up to high flow levels of 1 m s-1. The response time is of the order of milliseconds.

Abstract: Evidence of Hot Fluid Flow in Diapiric Area in Overpressured System of Yinggehai Basin, South China Sea 

Abstract: Evidence of Hot Fluid Flow in Diapiric Area in Overpressured System of Yinggehai Basin, South China Sea 

Origin of Dinantian zebra dolomites south of the Brabant‐Wales Massif, Belgium

Zebra dolomites, characterized by a repetition of dark grey (a) and light (b) coloured dolomite sheets building up abbabba‐sequences, occur in Dinantian strata from deep boreholes (> 2000 m) south of the Brabant‐Wales Massif in Belgium. These zebra dolomite sequences are several tens of metres thick. The dark grey dolomite sheets (a) consist of non‐planar crystals, 80–150 μm in diameter. These crystals display a mottled red–orange luminescence and are interpreted to be replacive in origin. The white dolomite sheets (b) consist of coarse crystalline nonplanar b1 dolomite, which evolves outwards into transparent saddle shaped b2 dolomite. The b1 dolomites possess a mottled red–orange luminescence similar to the a dolomites, while the saddle shaped b2 rims display red to dark brown luminescent‐zones. The b1 dolomites are possibly partly replacive and partly cavity filling. Their b2 rims display criteria typical for a cement origin. Locally, cavities exist between two succeeding white dolomite sheets. These cavities make up ≈5% of the zebra rocks and are locally filled by saddle shaped ankerite and/or xenomorphic ferroan calcite.Geochemical and fluid inclusion data (Th ≈ 120 °C) indicate a burial diagenetic origin for these zebra dolomites. The a and b1 dolomites are characterized by similar geochemical compositions and fluid inclusion data pointing toward a related origin. To explain the development of the zebra textures, suprahydrostatic pressures in conjunction with late Variscan tectonic compression are invoked. A model involving dolomitizing fluids expelled during the Variscan orogeny is proposed. An overpressured system is also invoked to explain the important porosity development, the creation of centimetre‐scale subvertical displacements of the zebra pattern and the microfractures affecting the b1–b2 dolomite crystals.

Subglacial hydrology in north-western germany during the last glaciation: groundwater flow, tunnel valleys and hydrological cycles

Numerical modelling of groundwater dynamics in an overpressured system of subglacial aquifers and aquitards under the marginal portion of the Scandinavian ice sheet in northwestern Germany has been coupled with calculations of basal meltwater production rates and with field evidence of meltwater erosional features. Largely fine-grained subglacial sediments with relatively low hydraulic conductivities had a capacity to drain only about 25% of all basal meltwater produced in the relevant subglacial catchment area. The rest of the meltwater was evacuated through tunnel valleys in spontaneous outburst events. Subglacial drainage cycles are proposed, in which each cycle begins with meltwater drainage through the substratum (groundwater flow), followed by water ponding at the ice/bed interface, and ends in catastrophic meltwater releases through tunnel valleys.

Distribution and Generation of the Overpressure System, Eastern Delaware Basin, Western Texas and Southern New Mexico: DISCUSSION AND REPLY

Three subsurface pressure systems have been identified in the Delaware basin: an upper normal pressure system, a middle overpressure system, and a lower normal pressure system. The overpressure system occurs in the eastern Delaware basin, covering six Texas and New Mexico counties. The depth of the overpressure system ranges from 3100 to 5400 m. The normal fluid pressure gradient is 0.0103 MPa/m in the eastern Delaware basin. The highest overpressure gradient, however, approaches 0.02 MPa/m, which is close to the lithostatic gradient of 0.0231 MPa/m. The overpressure system has a relatively flat top and a downwarped bottom. The maximum thickness of the overpressure system reaches about 2300 m at the depocenter and pinches out toward the edges. An area of excess pressure o curs within the system where the highest excess pressure reaches 60 MPa. Local underpressured areas due to production are found in the lower normal pressure system in the War-Wink field area. Overpressure in the eastern Delaware basin is mainly associated with Mississippian, Pennsylvanian, and Permian (Wolfcampian) shale sequences, which also are major source rocks in the basin. Initial sedimentation rates within the overpressure system range from 17 to 90 m/m.y. Corrected bottom-hole temperature measurements indicate that the geothermal gradient within the overpressure zone is 25.1°C/km, which is higher than the basin's average geothermal gradient of 21°C/km. Temperatures at the top and bottom of the overpressure system are about 80 and 115°C, respectively. This temperature range approximates the temperature of the average clay dehydration zone. The oil window in the War-Wink field is coincident with the overpressure system, which implies that hydrocarbon g neration and migration are active in the overpressure system. A two-stage overpressure model is proposed. Hydrocarbon maturation combined with mechanical compaction disequilibrium and clay dehydration are the initial causes for overpressure generation due to an abnormal increase of fluid volume and pore space. Subsequently, the increase in temperature due to a decrease of thermal conductivity and fluid migration within the preexisting overpressure system would reinforce further overpressuring due to the fluid thermal expansion.

Overpressure Retardation of Organic-Matter Maturation and Petroleum Generation: A Case Study from the Yinggehai and Qiongdongnan Basins, South China Sea

Three superimposed pressure systems developed in the Yinggehai Basin, South China Sea, as indicated by seismic data, well logs, and direct pressure measurements. The organic maturation profile is nonlinear, with three nonparallel segments that correspond to the shallow, normal-pressured system; the intermediate, overpressured system; and the deep, strongly overpressured system, respectively. The intermediate and deep overpressured systems have abnormally low R0 gradients. The organic maturity of these overpressured rocks is significantly lower than the maturity of normal-pressured source rocks in nearby wells with similar thermal histories and does not match the thermal histories of the rocks. Such an organic maturity anomaly is distinctly different from those aused by variation in activation energies, conductivity contrasts, and hydrologic effects, and is confirmed to be the result of overpressure retardation. The degree to which the organic-matter maturation is retarded, expressed as the difference between predicted and measured vitrinite reflectance, increases exponentially with increasing fluid pressure, confirming that pressure increases the activation energies of organic-matter maturation reactions. Overpressure retardation has been proven to be conditional and quite important for clearly understanding petroleum generation, migration, and accumulation in overpressured sedimentary basins.