Origin Of Overpressure Research Articles

Overpressure origin and evolution during burial in the shale gas plays of the Wufeng-Longmaxi formations of southern Sichuan basin

Identifying the origin, evolution and distribution of overpressure poses a significant challenge in shale gas reservoir, particularly in the deeper layers and zones of the Wufeng-Longmaxi Formations. To uncover the origin mechanism and reconstruct the evolution process of overpressure, samples are collected from the Wufeng-Longmaxi Formations in the southern Sichuan Basin. Subsequently, a combination of multi well-logging methods is employed to comprehensively identify the origin mechanism of overpressure. Additionally, forward numerical simulation and Laser Raman inversion validation are utilized to reconstruct the evolution process of overpressure during burial. Results show that: (1) Hydrocarbon generation pressurization is identified as the dominant origin mechanism of overpressure in the Wufeng-Longmaxi shale gas reservoir, employing a combination of well-logging methods, the revised Bowers method, and a velocity-density intersection chart. (2) The Wufeng-Longmaxi shale undergoes three main pressurization processes, including kerogen generation oil pressurization, crude oil cracking gas pressurization and kerogen cracking gas pressurization. Hydrocarbon generation pressurization correlates closely with factors such as organic matter abundance and type, original hydrogen index, kerogen mass fraction and transformation rate, oil cracking rate and retention coefficient. (3) The pressure coefficient of kerogen generation oil pressurization and gas cracking pressurization reaches 1.73 and 2.78, respectively. The contribution of crude oil cracking gas pressurization (75.6 MPa) is significantly greater than kerogen generation oil pressurization (37.8 MPa). (4) The density of methane inclusions (0.290–0.319 g/cm3) is corroborated through both Laser Raman spectroscopy and homogenization temperature analysis. The paleo pressure is recovered as 90.3–193.5 MPa, with a corresponding pressure coefficient spanning from 1.39 to 2.98. This study not only contributes to the advancement of the shale gas accumulation theory, but also provides valuable insights to guide the further exploration and development of marine shale gas within the intricate tectonic setting of the Sichuan Basin.

The Origin of Overpressure in the Pinghu Tectonic Zone of Xihu Depression and Its Relationship with Hydrocarbon Accumulation

Clarification of the cause of overpressure is of great significance for the study of hydrocarbon generation, migration and accumulation processes under an overpressure environment, and the prediction of predrilling pressure. The cause of overpressure in Xihu Depression is still controversial. This study, based on mudstone compaction research and organic matter correction of logging data, ascertained the causes of overpressure in the Pinghu tectonic zone, Xihu Depression. A modified discrimination method for overpressure genesis was used and the overpressure–hydrocarbon distribution relationship was explored. The results show the following: (1) Vertically, overpressure is located in the lower section of the Pinghu Formation, with a pressure coefficient of 1.1–1.7 and a pressure relief feature. On the plane, the pressure coefficient in the Pingzhong area exceeds that in the Pingbei area. (2) The main causes of abnormal high pressure in the research area are undercompaction and fluid expansion. The fluid expansion mechanisms include overpressure transmission and hydrocarbon charging within the reservoir as well as hydrocarbon generation pressurisation and overpressure transmission within the source rock. (3) In the surge section of kaolinite, under fluid pressure, kaolin migrates toward the low-pressure areas, decreasing the porosity in low-pressure areas and preserving pores in high-pressure areas. The evolution of fluid pressure can be divided into two stages: pressurisation; and both pressurisation and pressure relief. Pressurisation and pressure relief drive hydrocarbon charging, but hydrocarbons are more enriched in overpressure layers.

Introducing Novel Correction Methods to Calculate Sedimentary Basin Overpressure and Its Application in Predicting Pressure Value and Origin

Overpressure is an important phenomenon observed in sedimentary basins. To enhance the precision of identifying the origin of overpressure and predicting it accurately, this paper presents four practical methods for analyzing formation overpressure, taking into account various factors, such as burial history, rock properties, mechanical models and data quality. By illustrating the application examples in typical basins, this paper demonstrates the practicality and potential of each model. Based on the analysis of organic matter content and denudation, our understanding of the origin of overpressure in the first member of the Cretaceous Qingshankou Formation in Songliao Basin has been revised. A novel model based on the static equilibrium equation was established, which not only demonstrates theoretical rationality but also exhibits practical advantages in the Junggar Basin and Southeast Hainan Basin. The incorporation of lateral loading in the correction model and the utilization of colored inversion in seismic velocity processing model have significantly enhanced pressure prediction accuracy in Junggar Basin and Southeast Hainan Basin, respectively. In conclusion, these correction models demonstrate a high level of reliability and possess significant potential for widespread adoption.

Overpressure characteristics and geological origin of the paleogene in Baiyun Sag, Pearl River Mouth Basin

The accurate understanding of the origin of overpressure is the basis of formation pressure research and the core work of determining the distribution of formation pore pressure in drilling design. The deep water area of Baiyun Sag in the Pearl River Mouth Basin in the northern margin of the South China Sea has been proved to be a hydrocarbon rich depression. In recent years, overpressure has been encountered in the middle and deep layers, but the geological origin of the overpressure in the deep water area are still unclear. Therefore, the pore pressure profile of the Baiyun Sag was evaluated using the data of drilling and seismic velocity, and the relationship between overpressure and the generation mechanism was established. Most of the surrounding and shallow strata of Baiyun Sag show normal pressure, while the middle and deep strata generally show overpressure, with the maximum pressure coefficient up to 1.55. The overpressure zone mainly develops from the middle of Zhujiang Formation to the upper member of Enping Formation. The mudstone resistivity and density values of the two typical Wells deviated from the normal trend. The density showed a constant trend with the increase of depth, while the resistivity showed a certain degree of reversal. The upper layer of the overpressure zone showed abnormally high resistivity values, corresponding to the overgland and transition zone formed by high carbonate content. There is no obvious increase of porosity in the overpressured sandstone strata of Baiyun sag, which does not have the characteristics of undercompaction. The velocity in the overpressure section of the two Wells varies greatly, and the density is basically unchanged or slightly decreased. It is concluded that the overpressure is caused by fluid expansion from hydrocarbon-generation. The large-scale and long-term fluid expansion caused by hydrocarbon generation and expulsion of source rocks provides the power source for the formation of overpressure in Baiyun sag. The distribution of effective source rocks has a controlling effect on the distribution range of overpressure. The tight sandstone strata in the Zhuhai, Wenchang and Enping formations become the cover layer of the overpressure system in the longitudinal direction.

Overpressure origins and evolution in deep-buried strata: A case study of the Jurassic Formation, central Junggar Basin, western China

Overpressure is significant to the exploration and exploitation of petroleum due to its influence on hydrocarbon accumulation and drilling strategies. The deep-burial hydrocarbon reservoirs of Jurassic strata in the central Junggar Basin are characterized by intensive overpressure, whose origins are complex and still unclear. In this study, Bowers’ method and sonic velocity-density crossplot method based on well logging data were used as a combination for overpressure judgements in geophysics. Furthermore, the corresponding geological processes were analysed in quality and quantity to provide a rational comprehension of the overpressure origins and the model of overpressure evolution and hydrocarbon accumulation processes. The results showed that hydrocarbon generation in the Jurassic source rocks led to overpressure in the mudstones, while hydrocarbon generation in Permian source rocks led to overpressure in the sandstone reservoirs in Jurassic strata by vertical pressure transfer. The burial and thermal history indicated that the aquathermal effect of pore fluids by temperature increase in deep strata is also an important origin of overpressure, while disequilibrium compaction may not be the dominant cause for the overpressure in deep-buried strata. Furthermore, the continuous tectonic compression in both the north–south and west-east trends from the Jurassic period to the present may also have enhanced the overpressure in deep strata. Meanwhile, the developed faults formed by intensive tectonic compression led to pressure transfer from source rocks to the Jurassic reservoirs. Overpressured geofluids with hydrocarbons migrated to sandstone reservoirs and aggravated the overpressure in the Jurassic strata. To conclude, the intensive overpressure in the central Junggar Basin is attributed to the combination of multiple mechanisms, including hydrocarbon generation, the aquathermal effect, tectonic compression and pressure transfer. Furthermore, the developed overpressure indicated hydrocarbon migration and accumulation processes and the potential of oil and gas reservoirs in deeply buried strata. We hope this study will provide a systematic research concept for overpressure origin analysis and provide guidance for petroleum exploration and exploitation in deep-buried strata.

A multi-proxy approach to detect the pore pressure and the origin of overpressure in sedimentary basins: An example from the Gulf of Suez rift basin

The pore pressure gradient and fracture gradient (PPFG) are critical parameters for drilling mud weight design in the energy industry. Successful drilling operations can be achieved successfully through the understanding of the pore pressure and fracture pressure in the subsurface succession. The scope of this research is to use an integrated approach that encompasses well-logging, basin modeling, drilling-based interpretations, and reservoir measurement methods to gain a reasonable PPFG model and decrease the drilling uncertainties in the El Morgan oil field in the Gulf of Suez. Moreover, it investigates the overpressure generation mechanisms in the basin, which have not been studied before in this area. In this work, PPFGs of more than 16 km of cumulative thick sedimentary succession were modeled and evaluated using an integrated approach. This study utilizes Eaton’s sonic and resistivity-based methods for pore pressure evaluation, while vertical stress was calculated based on the composite density profile. The study revealed that the top geo-pressure was detected at a depth of 1,030 m of Tortonian sediments. Late Miocene sediments reveal hard over-pressure with a maximum gradient of 0.55 PSI/feet, while Middle Miocene sediments exhibit mildly over-pressured, normal, and sub-normal pore pressure zones. The lowest pore pressure values were measured in the Langhian-Serravalian Kareem reservoir with a gradient of 0.29 PSI/feet. With the exception of a slight difference in the reservoir section, the pore pressure profiles in the northern and southern parts of the El Morgan oil field are relatively similar. Reservoir connectivity is believed to be the main reason behind pore pressure magnitude differentiation in the Middle Miocene reservoirs. The key mechanism for generating overpressure has been identified as disequilibrium compaction, and reservoir overcharging may contribute as an excess-pressure generation mechanism at the reservoir level. The presented approach can be applied in PPFG studies for both development and exploratory geomechanical studies in other areas of the Gulf of Suez basin or elsewhere in the world.

Characteristics, origin and controlling effects on hydrocarbon accumulation of overpressure in foreland thrust belt of southern margin of Junggar Basin, NW China

Aiming at the differential distribution of overpressure in vertical and lateral directions in the foreland thrust belt in the southern margin of Junggar Basin, the study on overpressure origin identification and overpressure evolution simulation is carried out. Based on the measured formation pressure, drilling fluid density and well logging data, overpressure origin identification and overpressure evolution simulation techniques are used to analyze the vertical and lateral distribution patterns of overpressure, genetic mechanisms of overpressure in different structural belts and causes of the differential distribution of overpressure, and the controlling effects of overpressure development and evolution on the formation and distribution of oil and gas reservoirs. The research shows that overpressure occurs in multiple formations vertically in the southern Junggar foreland thrust belt, the deeper the formation, the bigger the scale of the overpressure is. Laterally, overpressure is least developed in the mountain front belt, most developed in the fold anticline belt, and relatively developed in the slope belt. The differential distribution of overpressure is mainly controlled by the differences in disequilibrium compaction and tectonic compression strengths of different belts. The vertical overpressure transmission caused by faults connecting the deep overpressured system has an important contribution to the further increase of the overpressure strength in this area. The controlling effect of overpressure development and evolution on hydrocarbon accumulation and distribution shows in the following aspects: When the strong overpressure was formed before reservoir becoming tight overpressure maintains the physical properties of deep reservoirs to some extent, expanding the exploration depth of deep reservoirs; reservoirs below the overpressured mudstone cap rocks of the Paleogene Anjihaihe Formation and Lower Cretaceous Tugulu Group are main sites for oil and gas accumulation; under the background of overall overpressure, both overpressure strength too high or too low are not conducive to hydrocarbon enrichment and preservation, and the pressure coefficient between 1.6 and 2.1 is the best.

Origins of overpressure in the central Xihu depression of the East China Sea shelf basin

The origins of overpressure in the central Xihu depression have been accepted to result from disequilibrium compaction and hydrocarbon generation. This study uses organic matter correction of the log data in source rocks and five empirical methods (the multilogging combination method, Bowers’ method, the sonic velocity–density cross-plot method, the porosity method, and the pressure inversion method) to understand the origins of overpressure in the central Xihu depression. Overpressure strata are mainly distributed in the following two areas: 1) the Pinghu Formation on the western slope of the central Xihu depression; 2) the Pinghu Formation and lower part of the Huagang Formation in the western sag and the central Inversion anticline belt. The areas contain two types of pressure profiles: normal pressure–overpressure and normal pressure–overpressure–normal pressure. At the western slope and the western sag, overpressure in source rock and nonsource rock was caused by hydrocarbon generation and pressure transfer, respectively. At the central inversion anticline belt, however, overpressure was caused by hydrocarbon generation and tectonic compression in source rock and by pressure transfer and tectonic compression in nonsource rock. We also propose criteria for confirming the combination of tectonic compression and hydrocarbon generation as the origin of overpressure, where the porosity distribution in the overpressured mudstone is consistent with the normal compaction trend or conforms to a higher level of normal compaction as the depth increases. In the sonic velocity–effective stress diagram, sonic velocity increases with decreasing effective stress, whereas density and sonic velocity both increase with increasing normal compaction in the density–sonic velocity diagram.

The significance of a piecemeal geometric model of mudstone compaction: Pinghu Slope, Xihu Depression, Eastern China

The geometric model of the loading curve used to express mudstone compaction behaviour profoundly influences the quantitative estimation of pore-pressure and identification of the origin of overpressure. The verification of field data from the Pinghu Slope Belt indicates that as burial depth increased, the compaction process was divided into three sections of mechanical compaction fitted by a convexity function, chemically enhanced mechanical compaction fitted by a concavity function, and stable compaction fitted by a convexity function. The piecemeal geometric model of mudstone compaction expressed by the loading curve is more appropriate than previous monotonous loading curves for characterising compaction behaviour, amplifying the effect of chemical compaction, and expressing chemical compaction behaviour. The piecemeal geometric model of mudstone compaction behaviour yields more precise results of formation pressure prediction and overpressure origin identification.

Horizontal tectonic stress as a cause of overpressure in the southern margin of the Junggar Basin, northwest China

Overpressure can be widely observed in the Jurassic, Cretaceous and Paleogene reservoirs in the southern margin of the Junggar Basin, northwest China. The study region was subjected to intense tectonic stress from the neighbouring North Tianshan Mountains since the Miocene. However, the causes of tectonic stress-induced overpressure are still poorly understood. In this study, we considered the plots of sonic velocity against density and vertical effective stress and applied numerical simulations and geological analysis to determine the origin of overpressure with respect to mudstone and sandstone in the area under study by investigating the compaction of mudstone and analysing the overpressure distribution. Our results indicate that the most overpressured mudstone in the second and third structural belts and the east of the Sikeshu Sag exhibits abnormally high porosity, which is obvious from the regional seal mudstone of the Paleogene Anjihaihe and Lower Cretaceous formations. The overpressure in these mudstones can be mainly attributed to the increase in vertical stress and horizontal tectonic stress (HTS). The current overpressures in the reservoir sandstone of the second and third structural belts as well as towards the east of the Sikeshu Sag are mainly induced by HTS, overpressure transfer and increase in vertical stress. The contribution of the overpressure associated with the generation of HTS to the total overpressure differs in different belts. This contribution ranges from 28.5% to 69.2%, 21.2%–46.2% and 16.3%–34.8% in the second, the third structural belts and the east of the Sikeshu Sag, respectively. Thus, HTS can be the primary contributor to the reservoir overpressure in many zones in the area under study.

Overpressure Origin, Quantitative Calculation and Evolution in the Upper Segment of the Xiaganchaigou Formation (E32) from the Western Qaidam Basin, Northwestern China

Overpressure Origin, Quantitative Calculation and Evolution in the Upper Segment of the Xiaganchaigou Formation (E32) from the Western Qaidam Basin, Northwestern China

Prediction of over pressured zones from well logs in the Niger delta

The changes of some geophysical properties such as sonic compressional velocity with depth is an indication of the pressure system of over-pressured zones. Once these anomalous pressures are not forecasted correctly before drilling, calamitous occurrences, for instance kicks, and blowouts may ensue. This paper investigated the distribution and origin of overpressures in the Niger delta sedimentary basin using well log data. A Normal Compaction Trend line was generated by means of sonic velocity data and the overpressure zones were detected by observing the reversals of the sonic velocity data. The outcome of the formation pressure gradient prediction infers that the commencement of overpressure in the study area occurs at 7500ft with a resultant pressure gradient of 0.51psi/ft. Meanwhile, the findings of the assessment of sonic velocity and density cross-plots revealed disequilibrium compaction as the primary cause of overpressure. The prominent zones of overpressure were detected to appear at depths between 11100ft to 11300ft with an associated pressure gradient of 0.52psi/ft. to 0.53 psi/ft. This work will help in precisely predicting zones of overpressure in the research area prior to drilling.

Overpressure origin and its effects on petroleum accumulation in the conglomerate oil province in Mahu Sag, Junggar Basin, NW China

The origin of overpressure and its effect on petroleum accumulation in the large Permian/Triassic conglomerate oil province in the Mahu Sag, Junggar Basin have been investigated based on comprehensive analysis of log curve combinations, loading-unloading curves, sonic velocity-density cross-plot, and porosity comparison data. The study results show that there are two kinds of normal compaction models in the study area, namely, two-stage linear model and exponent model; overpressure in the large conglomerate reservoirs including Lower Triassic Baikouquan Formation and Permian Upper and Lower Wu'erhe Formations is the result of pressure transfer, and the source of overpressure is the overpressure caused by hydrocarbon generation of Permian Fengcheng Formation major source rock. The petroleum migrated through faults under the driving of hydrocarbon generation overpressure into the reservoirs to accumulate, forming the Permian and Triassic overpressure oil and gas reservoirs. The occurrence and distribution of overpressure are controlled by the source rock maturity and strike-slip faults connecting the source rock and conglomerate reservoirs formed from Indosinian Movement to Himalayan Movement. As overpressure is the driving force for petroleum migration in the large Mahu oil province, the formation and distribution of petroleum reservoirs above the source rock in this area may have a close relationship with the occurrence of overpressure.

Formation of inter-salt overpressure fractures and their significances to shale oil and gas: A case study of the third member of Paleogene Shahejie Formation in Dongpu sag, Bohai Bay Basin

Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation (Es3) of Dongpu sag in Bohai Bay Basin as an example, the origin of overpressure, development characteristics, formation and evolution mechanism, formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification, scanning electron microscopy observation, analysis of fluid inclusions, logging data analysis, and formation pressure inversion. The results show that overpressure is universal in the salt-lake basin of Dongpu sag, and under-compaction caused by the sealing of salt-gypsum layer, pressurization due to hydrocarbon generation, transformation and dehydration of clay minerals, and fault sealing are the 4 main factors leading to the occurrence of overpressure. The overpressure fractures are small in scale, with an average length of 356.2 μm and an average underground opening of 11.6 μm. But they are densely developed, with an average surface density of 0.76 cm/cm2. Moreover, they are often accompanied by oil and gas charging, and thus high in effectiveness. Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging, approximately 25–30 Ma ago and 0–5 Ma ago. Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas. They contribute 22.3% to the porosity of shale reservoir and 51.4% to the permeability. They can connect fracture systems of multiple scales, greatly improving the quality of shale reservoir. During the development of shale oil and gas, inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures, giving rise to complex and highly permeable volumetric fracture networks, improving hydraulic fracturing effect and enhancing shale oil and gas productivity.

Distribution, origin, and evolution of overpressure in the Shahejie Formation of northern Dongpu Depression, Bohai Bay Basin, China

The distribution, origin, and evolution of overpressure in the petroliferous basin are important problems that need to be addressed for oil and gas exploration. The distribution and origin of overpressure in the Shahejie Formation in the northern Dongpu Depression are analyzed based on geological studies, logs, and pressure data. The contribution ratios of different overpressure origins are quantified, and the evolutionary stages of overpressures of different origins are further divided. The results show that the formation pressure coefficients of Shahejie Formation in the Dongpu Depression are mainly within the range of 0.9–1.5. The overpressures are mainly distributed in the Sha-3 and Sha-4 Members of the Haitongji sag, the Central uplift belt and the Qianliyuan sag. From the sag to its surrounding area, the formation pressure coefficient gradually decreases. The high deposition rate and strong hydrocarbon generation are the main causes of overpressure formation in the Shahejie Formation in the Dongpu Depression. Based on the stress–strain characteristics of different origin overpressures and the log response parameters, two models, acoustic travel time-effective vertical stress and electrical resistivity-effective vertical stress, are established to identify and quantify the different origin overpressures. The calculation results for the area from the Haitongji sag and Qianliyuan sag to the Central uplift belt show that the main cause of overpressure gradually changes from both disequilibrium compaction and hydrocarbon generation to disequilibrium compaction as the main factor, with the contribution of disequilibrium compaction to overpressure in the Central uplift belt at about 87%. The Sha-3 Members of the Shahejie Formation in the Haitongji sag and the Qianliyuan sag are more strongly affected by hydrocarbon generation, which accounts for 42% and 47.5% of overpressure origin, respectively. There are five stages in the evolution of overpressure in the Shahejie Formation in the northern Dongpu Depression: normal compaction (before 35 Ma), mixed pressurization (35-27 Ma), uplift and pressure release (27-17 Ma), disequilibrium compaction (17-11 Ma), and secondary mixed pressurization (12 Ma-present).

Advances in the origin of overpressures in sedimentary basins

Much progress in the studies on overpressuring mechanisms has been made during the past one to two decades. (1) The causes of overpressure are divided into five categories, namely, disequilibrium compaction, fluid expansion, diagenesis, tectonic compression and pressure transfer. The fluid expansion involves hydrocarbon generation, oil cracking to gas and hydrothermal expansion. The diagenesis includes smectite-to-illite transformation. (2) Six methods for identifying overpressure origin are proposed, including log curves combination analysis, Bowers method (loading-unloading diagram), velocity-density crossplotting, correlation of porosities, pressure calculation and correlation, and comprehensive analyses. (3) With more and more application of empirical methods in the study of overpressure formation, almost all of the overpressure cases that are traditionally thought to be caused by disequilibrium compaction are denied totally or partly. Instead, the hydrocarbon generation is demonstrated to be the most significant mechanism for overpressure formation; the clay diagenesis (especially the smectite-illite transformation) as well as tectonic compression and pressure transfer are also important for overpressure formation. In addition, the overpressure formation in many basins is thought to be influenced by the combination of two or more overpressuring mechanisms. (4) Causes of overpressuring differ in lithology; for mudstones, the overpressure formation in source rocks is usually different from that of non-source rocks, the former of which is frequently related to hydrocarbon generation and sometimes also affected by diagenesis, while the later of which is commonly related to disequilibrium compaction, diagenesis and pressure transfer; for permeable rocks such as sandstones, overpressure is mainly caused by pressure transfer. (5) Because organic matter has an obvious influence on logging parameters such as density and acoustic velocity, an appropriate correction on the content of organic matter is needed when these logging data are used to analyze overpressure formation in organic-rich mudstones. It has been revealed that the cause of overpressuring based on the corrected log data can be quite different from that without correction.

Typical disequilibrium compaction caused overpressure of Paleocene Dongying Formation in northwest Liaodongwan Depression, Bohai Bay Basin, China

Bohai Bay Basin is an important petroliferous basin in China. Petroleum potential in Liaodongwan Depression of offshore Bohai Bay Basin has been proved by previous exploration and production, and overpressures were encountered. Summarized by the DST measurements and mud weight data of over 60 wells in Liaodongwan Depression, the abnormally high pressures preserved in Paleocene Dongying (Ed) Formation distribute over the entire depression and could reach as high as 64.2MPa around the depth of 3420m. Through the well logs analysis, typical corresponding characteristics to disequilibrium compaction can be found: higher in acoustic transit time and lower in density than normal compaction trend. Based on these corresponding features the pore pressure prediction was carried out for all the wells in the area. Sonic velocity-effective stress relationship and sonic velocity-density cross plots were employed to investigate the origin of overpressure in Ed Formation. Sedimentation rates and geothermal features were studied as well. Combining all the analysis, the overpressure of Ed Formation in northwest Liaodongwan Depression was dominantly generated by disequilibrium compaction. With this widely distributed overpressure, thick layers of mudstone of Ed Formation serve as a regional overpressured caprock.

Reevaluation of the origin of overpressure in the inter-salt shale-oil reservoir in Liutun Sag, Dongpu Depression, China

The inter-salt shale-oil reservoir in Liutun Sag of the Dongpu Depression in the Bohai Bay Basin, China, has a strong overpressure and produces abundant heavy oil with little water. The previous researches claimed that the overpressure was mainly a result of disequilibrium compaction, which seemingly contradicts with the little water production. More systematic work is required to understand the origin of the overpressure and the oil/water production. Formation testing, mud weight and wireline-log data were used to recognize overpressure and equivalent depth method and sonic transit time-density crossplot were used to identify the overpressure mechanisms. In addition, the tectonic evolution was included in the analysis, aiming at understanding the oil/water production features. Disequilibrium compaction is considered as the main overpressure mechanism, due to the sealing of salt rocks. Hydrocarbon generation, which mainly generated low maturity heavy oil, contributes to the overpressure feature with the help of tectonic uplift. The overpressure evolution, which corresponds to the tectonic evolution, consists of three phases: early disequilibrium compaction, middle fluid substitution and late disequilibrium compaction. The tectonic uplift in the middle phase, it is proposed, contributes most to the little water production. With the tectonic uplift, a lot of fractures formed, which would promote oil and water drain. However, due to the low mobility of heavy oil, more water tended to be drained away from the region of strong overpressure, and thus the intuitive fluid substitution occurred. The findings above indicate that the exploration of inter-salt shale-oil reservoirs in Liutun Sag or even the other areas of Dongpu Depression may additionally consider some special strategies, including the occurrence of salt rocks and the hybrid overpressure feature of disequilibrium compaction and fluid expansion besides tectonic uplift.

Origin of over-pressure in clastic rocks in Yuanba area, northeast Sichuan Basin, China

Abnormally high pressures are widely developed in clastic formations with burial depths greater than 3500 m in Yuanba area, where the top depth of over-pressure zone encompasses different formations in different locations. With increasing depths, well logs response in mudstone to over-pressure zone show that no significant anomalies occur in sonic and density logs, suggesting that the over-pressured mudstones are normally compacted. Multiple evidences from well-log responses to over-pressure, low porosity, normal geothermal gradient, low sedimentary rate and velocity-density crossplots show that over-pressures in mudrocks were created by gas generation and not related to the disequilibrium compaction. Based on diagenetic evolution analysis, stratigraphic framework and gas accumulation model, pressure transference and oil-to-gas cracking are expected to be the main causes of over-pressure in sandstone. Because of the favorable sealing condition and gas-bearing properties, lateral tectonic compression has also been an important factor for over-pressure development and preservation since early Cretaceous.

Origin of Overpressure in a Field in the Southwestern Malay Basin

Summary This study demonstrates the use of wireline logs for the overpressure-mechanisms analysis in a field in the southwestern Malay basin. The development of overpressure means that the fluid movement in the pores is retarded, both vertically and laterally. In many Tertiary basins, overpressure is mainly generated by compaction disequilibrium caused by a high deposition rate and low permeability in shales. In the Malay basin, temperature and high-heat flow also play an important role in generating overpressure at a shallow depth, because the geothermal gradient is very high (40–60°C/km). Pore-pressure profiles and crossplots of sonic velocity/vertical effective stress and of velocity/density are used to derive the overpressure-generating mechanisms. The results obtained from the crossplots of 10 wells reveal that in the study area, overpressure is generated by both primary (compaction-disequilibrium) and secondary (fluid-expansion) mechanisms. The overpressure-magnitude analysis suggests that the overpressure generated by the secondary mechanism is very high compared with the primary mechanism. In all the wells, the Eaton (1972) method with an exponent of 3 gives good prediction when overpressure is the result of the compaction-disequilibrium mechanism, but it underpredicted the high pore pressure where the fluid-expansion mechanism is also present. However, by use of a higher Eaton exponent of 5 for the fluid-expansion mechanism, the overpressures are predicted quite well. The Bowers (1995) method, by use of the unloading parameter (U) of 6, is also used for pressure prediction and it gives a reasonably good prediction in the high-overpressure zone of all the wells.