Eastern Pearl River Mouth Basin Research Articles

Hydrocarbon accumulation mechanism in the far-source reservoirs of Dongsha Uplift of the Pearl River Mouth Basin, northern South China Sea

There are a lot of exploration examples of far-source reservoirs, but the description of the connotation and accumulation mechanism is very rare. The Dongsha Uplift is one of the major oil-producing areas in the eastern Pearl River Mouth Basin in the northern South China Sea. The uplift lacks generally hydrocarbon source rock and is far from the generative kitchen, making it a typical far-source oil and gas reservoir. The accumulation mechanism of crude oil in the reservoirs are still unknown. By systematically comparing biomarkers, nitrogenous compounds and isotopic characteristics of the crude oil and source rocks with those in the neighboring depressions, and combining with fracture, sand body, unconformity migration conduits and barrier conditions, this paper simulates oil and gas migration paths and formation time with Pathway and IES software, and analyzes oil and gas accumulation process from a dynamic perspective. We found that the deep lacustrine hydrocarbon source rocks of the Wenchang Formation in the H26 Sag of the Huizhou Depression reached peak oil production at the end of the Hanjiang Formation deposition. Influenced by the strong tectonic activities at the final deposition of the Hanjiang and Yuehai formations, two episodes of hydrocarbon charging occurred at the top of the Dongsha Uplift. The accumulation of hydrocarbons far from the source rocks in the Dongsha uplift is mainly controlled by the efficient carrier system at hydrocarbon generation period. The oil is mainly accumulated in L4-1 and L11-1 oil fields through the long-distance stepped migration mode. The spatial and temporal relationship between hydrocarbon generation of source rock and episode of fault activity are mainly responsible for accumulation in the far-source reservoirs of the Dongsha Uplift.

Application of Microbiological Method to Hydrocarbon Exploration in Eastern Pearl River Mouth Basin

Application of Microbiological Method to Hydrocarbon Exploration in Eastern Pearl River Mouth Basin

Analysis on the causes of failed wildcat wells in the eastern Pearl River Mouth Basin and its enlightenment

Analysis on the causes of failed wildcat wells in the eastern Pearl River Mouth Basin and its enlightenment

Geomorphological-geological-geophysical signatures of high-flux fluid flows in the eastern Pearl River Mouth Basin and effects on gas hydrate accumulation

Marine hydrate reservoirs can be divided into focused high-flux and distributed low-flux gas hydrate systems according to free gas migration control mechanisms. In focused high-flux hydrate reservoirs, fluids easily break through the pressure of overlying sediments and reach the shallows, creating a series of geomorphological-geological-geophysical anomalies at and near the seafloor. Based on detailed interpretation of pre-drilling data in the eastern Pearl River Mouth Basin (PRMB), many anomalies related to the high-flux fluid flow are found, including seafloor mounds with intrusive characteristics, bright spot reflections above the bottom-stimulating reflector (BSR), phase reversals in the superficial layer, and an efficient fluid migration and accumulation system composed of fractures and uplifts. The second hydrate drilling expedition was carried out in the eastern PRMB in 2013 to study these anomalies. The acquired data show that high-flux fluid flow occurred in these sites. Gas hydrate pingoes, bright spot reflection above the BSR, and an efficient fluid migration and accumulation system can be used as identification signatures for high-flux fluid migration. The modes of high flux fluid flow are different in deep and shallow sediments during upward migration of fluid. Gas dissolved within migrating water dominates deep fluid migration and upward migration of a separate gas phase dominates the shallow process. This difference in migration models leads to formation of upper and lower concentrated hydrate reservoirs in the drilling area. The discovery of signatures of high-flux fluid flow and their migration modes will help with site selection and reduce risk in gas hydrate drilling.

Geochemical characteristics, origins, and model of lacustrine source rocks in the Zhu 1 depression, eastern Pearl River Mouth Basin, South China Sea

Variations in environmental and biological origins contributing to the heterogeneity of lacustrine source rocks can be illustrated in their diverse genetic facies. The Zhu 1 depression, eastern Pearl River Mouth Basin, South China Sea, is characterized by two thick, Paleogene, organic-rich synrift units, the Wenchang and Enping Formations. The integration of bulk geochemical and biomarker data with tectonic and sedimentary information provides the basis for a comprehensive assessment of the environmental and ecological changes through geologic time and their impact on the heterogeneity of these lacustrine source rocks. Both the Wenchang and Enping Formations display wide variations in total organic carbon content and hydrogen index values as well as biomarker composition, suggesting lateral and chronological changes in organic facies. Using gas chromatography–mass spectrometry and hierarchical cluster analysis, five genetic facies were identified within these two source horizons. These facies represent different organic-matter inputs and sedimentary and early diagenetic environments based on their distinctly different assemblages of 11 source-dependent biomarker ratios. Four facies were distinguished in the Wenchang Formation, and two facies were distinguished in the Enping Formation, with one being common to both formations. During the middle Eocene, the Wenchang Formation was deposited in a series of small, deep lakes of laterally variable salinity, acidity, and biofacies. During the deposition of the Enping Formation in the late Eocene and early Oligocene, the previous lakes merged into fewer lakes with shallower depth and larger areal coverage, with the biota becoming more uniform across the whole depression. The coevolution of these lacustrine settings and their biota is closely associated with the development of the Zhu 1 depression, within which multiple separate sags produced by rapid mid-Eocene subsidence finally merged into a single depositional unit during slow subsidence in the late Eocene and early Oligocene. Accordingly, an integrated model was established to provide an overview of the contrasting origins of lacustrine source rocks during the two Paleogene epochs. This model may have important implications for source-rock prediction in the undrilled parts of the basins or for reference to source-rock heterogeneity in other rift basins.

Cretaceous–Eocene provenance connections between the Palawan Continental Terrane and the northern South China Sea margin

The plate kinematic history of the South China Sea opening is key to reconstructing how the Mesozoic configuration of Panthalassa and Tethyan subduction systems evolved into today's complex Southeast Asian tectonic collage. The South China Sea is currently flanked by the Palawan Continental Terrane in the south and South China in the north and the two blocks have long been assumed to be conjugate margins. However, the paleogeographic history of the Palawan Continental Terrane remains an issue of uncertainty and controversy, especially regarding the questions of where and when it was separated from South China. Here we employ detrital zircon U–Pb geochronology and heavy mineral analysis on Cretaceous and Eocene strata from the northern South China Sea and Palawan to constrain the Late Mesozoic–Early Cenozoic provenance and paleogeographic evolution of the region testing possible connection between the Palawan Continental Terrane and the northern South China Sea margin. In addition to a revision of the regional stratigraphic framework using the youngest zircon U–Pb ages, these analyses show that while the Upper Cretaceous strata from the Palawan Continental Terrane are characterized by a dominance of zircon with crystallization ages clustering around the Cretaceous, the Eocene strata feature a large range of zircon ages and a new mineral group of rutile, anatase, and monazite. On the one hand, this change of sediment compositions seems to exclude the possibility of a latest Cretaceous drift of the Palawan Continental Terrane in response to the Proto-South China Sea opening as previously inferred. On the other hand, the zircon age signatures of the Cretaceous–Eocene strata from the Palawan Continental Terrane are largely comparable to those of contemporary samples from the northeastern South China Sea region, suggesting a possible conjugate relationship between the Palawan Continental Terrane and the eastern Pearl River Mouth Basin. Thus, the Palawan Continental Terrane is interpreted to have been attached to the South China margin from the Cretaceous until the Oligocene oceanization of the South China Sea. In our preferred paleogeographic scenario, the sediment provenance in the northeastern South China Sea region changed from dominantly nearby Cretaceous continental arcs of the South China margin to more distal southeastern South China in the Eocene.

Gas hydrate accumulation and saturations estimated from effective medium theory in the eastern Pearl River Mouth Basin, South China Sea

Pore- and fracture-filling gas hydrates were identified from the core samples at several sites during the second Guangzhou Marine Geological Survey (GMGS2) expedition. Well logs indicated that gas hydrate occurred in three distinct layers at site GMGS2-08. The gas hydrate saturations calculated from well-log data and the seismic responses for the three gas hydrate-bearing layers, especially within the middle carbonate layer, were poorly known. We estimated gas hydrate saturations using isotropic and anisotropic models based on the mineral composition of the sediments and the effective medium theory. In the upper and lower gas hydrate-bearing layers, saturations estimated from anisotropic models are close to those estimated from pressures cores and chlorinity data. The average saturation using an anisotropic model in the upper (fracture-dominated) hydrate layer is approximately 10% with a maximum value of 25%. In the lower (fracture-dominated) layer, the horizontal and vertical gas hydrate-filled fractures and visible gas hydrate were formed with a maximum saturation of approximately 85%. For the middle layer, well logs show high P-wave velocity, density, high resistivity as well as low gamma ray, porosity, and drilling rate, together indicating a carbonate layer containing gas hydrate. The hydrate saturations calculated from isotropic models assuming hydrate formed at grain contacts are less than 20%, which fit well with two values calculated from chlorinity data for this layer. The upper gas hydrate layer shows no clear seismic response and probably consisted of small fractures filled with gas hydrate. The middle carbonate and lower fracture-filled gas hydrate-bearing layers show pull-up reflections, with the carbonate layer exhibiting relatively higher amplitudes. Pore-filling gas hydrate was also identified just above the depth of the bottom-simulating-reflector (BSR) from the GMGS2-05 drill site. Below the BSR, the push-down reflections, polarity reversal, and enhanced reflections indicate the occurrence of free gas in the study area.

Rapid post-rift tectonic subsidence events in the Pearl River Mouth Basin, northern South China Sea margin

Data from 26 drill wells and 27 regional seismic profiles were integrated to investigate the timing, phase and origin of the post-rift subsidence in the middle to eastern Pearl River Mouth Basin (PRMB) of the northern South China Sea using the traditional 1-D backstripping technique. Different from previous research of backstripped tectonic subsidence in the basin, we calculated the tectonic subsidence using the newly built local porosity-depth relationships for decompaction and updated sedimentological and paleontological data for paleobathymetry reconstruction. Well-data based subsidence curves reveal a roughly decaying pattern in both the magnitude and rate of the post-rift subsidence in the PRMB, which is in accordance with the general decreasing trend of the thermal subsidence typical of a passive margin. Two events of rapid post-rift tectonic subsidence were identified, which occurred in the Early to early Middle Miocene and the Pliocene. The timing of the first rapid post-rift subsidence event varies and is earlier in the southern rather than northern part of the basin. Additionally, the amplitude of contemporaneous tectonic subsidence is greater in the southern part of the basin. The second rapid tectonic subsidence event occurred simultaneously in both the southern and northern parts of the basin, with the amplitude of subsidence being much greater in the southern part. We associate the first rapid subsidence event with the southward jump of the South China Sea spreading ridge, which occurred between the Oligocene and Early Miocene, while the second event with the arc-continent collision at Taiwan since the latest Late Miocene. The southern PRMB in the deep-water slope area shows a much higher magnitude of tectonic subsidence in both events than its northern counterpart in the shelf area, which could be associated with its much thinner lithosphere. The latter could cause upswelling of denser lower crust and upper mantle material, resulting in more rapid tectonic subsidence in the southern PRMB.

Texture and tectonic attribute of Cenozoic basin basement in the northern South China Sea

Based on the drilling data, the geological characteristics of the coast in South China, and the interpretation of the long seismic profiles covering the Pearl River Mouth Basin and southeastern Hainan Basin, the basin basement in the northern South China Sea is divided into four structural layers, namely, Pre-Sinian crystalline basement, Sinian-lower Paleozoic, upper Paleozoic, and Mesozoic structural layers. This paper discusses the distribution range and law and reveals the tectonic attribute of each structural layer. The Pre-Sinian crystalline basement is distributed in the northern South China Sea, which is linked to the Pre-Sinian crystalline basement of the Cathaysian Block and together they constitute a larger-scale continental block—the Cathaysian-northern South China Sea continental block. The Sinian-lower Paleozoic structural layer is distributed in the northern South China Sea, which is the natural extension of the Caledonian fold belt in South China to the sea area. The sediments are derived from southern East China Sea-Taiwan, Zhongsha-Xisha islands and Yunkai ancient uplifts, and some small basement uplifts. The Caledonian fold belt in the northern South China Sea is linked with that in South China and they constitute the wider fold belt. The upper Paleozoic structural layer is unevenly distributed in the northern South China. In the basement of Beibu Gulf Basin and southwestern Taiwan Basin, the structural layer is composed of the stable epicontinental sea deposit. The distribution areas in the Pearl River Mouth Basin and the southeastern Hainan Basin belong to ancient uplifts in the late Paleozoic, lacking the upper Paleozoic structural layers. The stratigraphic distribution and sedimentary environment in Middle-Late Jurassic to Cretaceous are characteristic of differentiation in the east and the west. The marine, paralic deposit is well developed in the basin basement of southwestern Taiwan but the volcanic activity is not obvious. The marine and paralic facies deposit is distributed in the eastern Pearl River Mouth Basin basement and the volcanic activity is stronger. The continental facies volcano-sediment in the Early Cretaceous is distributed in the basement of the western Pearl River Mouth Basin and Southeastern Hainan Basin. The Upper Cretaceous red continental facies clastic rocks are distributed in the Beibu Gulf Basin and Yinggehai Basin. The NE direction granitic volcanic-intrusive complex, volcano-sedimentary basin, fold and fault in Mesozoic basement have the similar temporal and spatial distribution, geological feature, and tectonic attribute with the coastal land in South China, and they belong to the same magma-deposition-tectonic system, which demonstrates that the late Mesozoic structural layer was formed in the background of active continental margin. Based on the analysis of basement structure and the study on tectonic attribute, the paleogeographic map of the basin basement in different periods in the northern South China Sea is compiled.

Genetic pattern of belt-wide petroliferous phenomenon in the eastern Pearl River Mouth Basin and its practical application

The Pearl River Mouth Basin (PRMB) covers an area of approximately 20×104 km2. However, oil-gas fields detected in this area thus far are highly concentrated and controlled predominantly by second-order structural belts, the seven largest of which aggregate proved oil reserves of 7.7×108 m3, accounting for 86% of the total discovered reserve in the basin. These second-order structures have one common phenomenon: oil is contained in all traps present in them. In other words, they are all belt-wide petroliferous reservoirs. Research has identified eight types of second-order structural belts under two categories in the eastern PRMB. Their petroliferous properties are subject to three typical constraints: petroliferous properties of subsags hosting these structural belts, locations of these belts in the petroleum system, and availability of traps prior to the hydrocarbon expulsion and migration. The formation and distribution of oil reservoirs in these belts are characterized by subsag-belt integration and “three-in-one”. The former indicates that sags and the second-order structural belts within the supply range of the sags constitute the basic units of hydrocarbon accumulations and are therefore inseparable. The latter indicates that a belt-wide petroliferous second-order structural belt always contains three important elements: hydrocarbon richness, effective pathway and pre-existing traps.

Preferential petroleum migration pathways in eastern Pearl River Mouth basin, offshore South China Sea

Secondary petroleum migration in the eastern Pearl River Mouth basin was modeled using the three-dimensional PATHWAYSTM model, which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces. The modeling results have accurately predicted the petroleum occurrences. Most commercial petroleum accumulations are along the predicted preferential petroleum migration pathways (PPMP), and most large fields (petroleum reserves greater than 1×108 t) have more than one preferential petroleum migration pathways to convey petroleum to the traps. The lateral migration distance for oil in the LH11-1 field, the largest oilfield so far discovered in the Pearl River Mouth basin, was more than 80 km. The case study suggests that in lacustrine fault basins, petroleum can migrate over a long distance to form large oilfields without driving force from groundwater flow. The focusing of petroleum originating from a large area of the generative kitchens into restricted channels seems to be essential not only for long-range petroleum migration in hydrostatic conditions, but also for the formation of large oil or gas fields. The strong porosity and permeability heterogeneities of the carrier beds and the relatively high prediction accuracy by a model that does not take into consideration of the effect of heterogeneity suggest that the positions of petroleum migration pathways in heterogeneous carrier beds with relatively large dipping angles are determined primarily by the morphology of the sealing surfaces at regional scales.

Characteristics and hydrocarbon potential of Mesozoic strata in eastern Pearl River Mouth basin, northern South China Sea

The well LF35-1-1 in the eastern Pearl River Mouth basin (PRMB) of the northern South China Sea revealed unmetamorphosed Middle-Late Jurassic neritic-bathyal sediments and Cretaceous fluvial-lacustrine sediments. Three tectonic movements were identified in Late Jurassic to Early Cretaceous, late Early Cretaceous, and Late Cretaceous to Paleocene, respectively. The Late Jurassic marine facies mainly contain the hydrocarbon source and reservoir-seal assemblages, providing a main exploration target.

04/00073 Geochemistry of petroleum systems in the eastern Pearl River Mouth Basin: evidence for mixed oils: Zhang, S. et al. Organic Geochemistry, 2003, 34, (7), 971–991

04/00073 Geochemistry of petroleum systems in the eastern Pearl River Mouth Basin: evidence for mixed oils: Zhang, S. et al. Organic Geochemistry, 2003, 34, (7), 971–991

Geochemistry of petroleum systems in the eastern Pearl River Mouth Basin: evidence for mixed oils

Detailed analyses of crude oils and source rock extracts indicate the presence of two effective petroleum source rocks in the eastern Pearl River Mouth Basin of the South China Sea region. Oils derived from the lacustrine source rocks in the Eocene Wenchang Formation are characterized by abundant C 30 4-methyl steranes with relatively few C 19 tricyclic terpane and bicadinanes. In contrast, oils from the Eocene-Oligocene Enping Formation source typically contain significant amounts of C 19 tricyclic terpane and bicadinanes, with relatively few 4-methyl steranes and high pristane/phytane ratios. However, the chemical compositions of a large number of oils do not conform to those of the unknown source rocks, consistent with mixing of the two sources. Mixed oils occur mainly in the Huizhou Sag and along its south margin, and the highest proportion of the Enping Formation source in the reservoired mixed oils is estimated to be around 80%. Laboratory mixing experiments using selected end-member oils indicate that, even with 50–80% contribution from the Enping Formation source, the mixtures still display sterane biomarker ratio signatures diagnostic of the Wenchang Formation source. Therefore, the presence of abundant 4-methyl steranes is a necessary but not sufficient indicator for the Wenchang Formation source in the eastern Pearl River Mouth Basin. Although our results support the Wenchang Formation as a major source to the known oil discoveries in the study area, the contribution of the Enping Formation source to traps in the Huizhou Sag and surrounding area cannot be ignored. There is clear evidence for the recharge of Enping Formation derived oils into early-biodegraded oil accumulations to form light oil accumulations.

Application of Sequence Stratigraphy to Exploration and Development in Eastern Pearl River Mouth Basin, South China Sea: ABSTRACT

Some appropriate conditions for applying sequence stratigraphy in PRMB exist: (1) The PRMB is a Cenozoic extension basin with weak tectogenesis after rifting; (2) The late Tertiary successive sediments deposited over the area from coast to deep marine basin are well developed and their seismic reflection is good in quality; (3) High resolution quantitative biostratigraphy has provided more precise data of eustasy. Twenty-two third order sequences since 30Ma were identified and are correlative to global cycle chart, but TB2.1 can be further divided into two third-order sequences. The 22 sequences from a [open quote]retrograding stacking pattern[close quote] which resulted in no large constructive delta or delta-related rollover anticlines. However, widely deposited seal rock possibly sealed the incised-valley, basin floor fan (BPF), and transgressive sandstone to form valid stratigraphic traps. Stratigraphic traps are particularly important in future exploration because of poor anticline trap types in the basin. Some ER stratigraphic traps can be predicted after studying the distribution of systems ER tracts in each sequence as a basic exploration unit. The BPF of TB2.1 which is closed to the source kitchen area could form a large subtle trap. Using the parasequence as a basic unit in reservoir scale will provide amore » new method to discover new reservoirs. For example, new oil reserves within the predicted stratigraphic trap related to K22, which can be divided into several parasequences in C.A. 16/08, were obtained after drilling. Sequence stratigraphy has become a valid high resolution tool for chronostratigraphic correlation and fine-division and exploration of stratigraphic traps.« less

A geochemical investigation of crude oils from Eastern Pearl River Mouth Basin, South China Sea

A thorough petroleum exploration of the Pearl River Mouth Basin (PRMB), South China Sea, began in 1983. At present, several oilfields have been found in the PRMB, mainly distributed in Dongsha Massif, Huizhou and Xijiang Depressions as well as Huilu Lowhigh, and one of them has been developed recently. The crude oils found in the basin can be classified into two types. One is normal waxy type, and the other is cyclic type, which may be caused by minor biodegration and is restricted to the Liuhua District of Dongsha Massif. However, on the basis of geochemical characteristics, all the crude oils are thermally mature, indicating that they are derived from source rocks which have entered the main oil generation period but their maturity is not high enough to reach the overmature stage. Moreover, in the biomarker distribution, the oils also share many similarities. Almost all the oils contain abundant C 30 4-methylsteranes with 24-ethyl side chain, ubiquitous oleanane and lower concentration of gammacerane, and possess high ratios of Ni/V, pristane to phytane and C 30 hopane over total C 29 steranes as well as high paraffin wax and low sulphur content, indicating that they originated from terrestrial organic matter deposited in lacustrine and marsh coal-forming environments. However, some characteristics resemble Brazilian offshore oils of salinewater lacustrine environment. The oils found in the PRMB can also be classified into three main genetic types based on the relative values of pristane over phytane ratio, C 29 sterane preference and the composition of the carbon isotope. Type I oils occurred in the Huizhou and the Xijiang Depressions and their adjacent Dongsha Massif. It has higher ratios of pristane to phytane (1.80–5.54 and 3.21 on the average scale) and heavier carbon isotopic composition, indicating that their source rocks contain much more abundant terrestrial higher plant input. Type II, encountered in Huilu Lowhigh and its bounding area of Dongsha Massif, has moderate ratios of pristane to phytane (1.78–2.94 and 2.39) and C 29 sterane similar to Type I. Type III is distributed in the Liuhua District of Dongsha Massif and has lower pristane to phytane ratios (ranging from 1.49 to 1.97), smaller C 29 sterane preference, higher C 30 4-methylsterane content and lighter carbon isotopic composition (less than −29%) even though they have undergone minor biodegration, implying that the source rocks of the oils are enriched with lacustrine organism input.

Geology and geochemistry of source rocks of the Eastern Pearl River Mouth Basin, South China Sea

The Pearl River Mouth Basin (PRMB), with mostly Cenozoic sediments, is one of the largest offshore oil-bearing basins in the South China Sea. It covers an area of 175,000 km 2 with the maximum sedimentary thickness of 12 km. Large structural belts trending NE-SW were developed in the basin. From north to south they are the Northern Fault Terrace-Uplift, Northern Depression, Central Uplift and Southern Depression Belts, respectively. The evolution of the PRMB had been in two stages, i.e. first faulting (from Paleocene to Early Oligocene) and then subsiding (starting from Late Oligocene), and the so-called Double Structural Layers were thus developed. Therefore, the sedimentary rocks of the PRMB are characterized by terrestrial followed by marine facies. Three terrestrial (accendingly known as Shenhu, Wenchang and Enping Formations, respectively) and six marine formations (accending named Zhuhai, Zhujiang, Yuehai, Wanshan and Quaternary Formations, respectively) were deposited. Geochemical statistical study of possible source rocks in the PRMB showed that both the terrestrial Enping and Wenchang Formations were richer in organic matter while both the marine Zhujiang and Zhuhai Formations are relatively poor. The type of organic matter is dominantly composed of Types II to I for the Wenchang Formation and mainly Type III for the other three overlying formations. Furthermore, in thermal maturity, the Zhujiang Formation is either immature or marginally mature, Zhuhai, marginally mature-mature, Enping, mature-highly mature and Wenchang, highly-overmature. Hence, the principal source rocks in the PRMB are the Wenchang and Enping Formations, and the Zhuhai and Zhujiang Formations could also be oil sources. Geochemical investigations indicated that the source rocks in the PRMB are characterized by lower saturates to aromatics ratio, commonly found 18a(H)-oleanane and resin-derived “W” and “T” compounds in some source rocks, enriched high molecular weight n-paraffin hydrocarbons and C29 steranes except for those of Wenchang and partly of the Enping Formations and predominant C30 hopane for Wenchang Formation in contrast to the others, which contained either predominant C31 homohopane or C29 norhopane or C27 trisnorhopane. However, the more abundant C30 4-methyl steranes discovered in crude oils of the PRMB were only identified from a thin setting of Wenchang Formation occurring in Huilu Lowhigh. Therefore, it was deduced that there must have been a certain setting of the Wenchang Formation which contains enriched C30 4-methylsteranes which are widely distributed in the depositional centers of the separated Depressions of the PRMB. They are possible oil sources of the reservoirs found in the PRMB.