Charge History Research Articles

Assessing the maturity of oil trapped in fluid inclusions using molecular geochemistry data and visually-determined fluorescence colours

The thermal maturity of oils extracted from inclusions and the fluorescence colours of oil-bearing fluid inclusions have been measured in 36 sandstone samples from Australasian oil fields. The inclusion oils were analysed using an off-line crushing technique followed by GC–MS. A maturity assessment was made for each inclusion oil using 25 molecular maturity ratios, including a newly defined dimethyldibenzothiophene ratio (DMDR). Each inclusion oil was placed in one of 4 maturity brackets, approximately equivalent to early, mid, peak and post oil generation windows. The fluorescence colours of oil inclusions were visually-discriminated into “blue”, “white” and “yellow plus orange” and their proportions estimated using point counting techniques. Sixteen samples have >85% of oil inclusions with blue fluorescence, whilst other samples have more variable fluorescence colours. One sample has 100% of oil inclusions with yellow plus orange fluorescence. The results show that samples containing mainly blue-fluorescing oil inclusions have thermal maturities anywhere within the oil window. In particular, the molecular geochemical data strongly suggests that oil inclusions with blue fluorescence can have relatively low maturities (calculated reflectance <0.65%), contrary to the widely applied assumption that blue fluorescence colours indicate high maturities. Samples containing mainly white-fluorescing oil inclusions have maturities anywhere within the oil window and cannot be distinguished using molecular geochemical parameters from samples containing mainly blue-fluorescing oil inclusions. Though few in number, samples with mainly yellow and orange-fluorescing oil inclusions tend to have maturities in the lower half of the oil window. The data presented strongly suggest that although the relationship between API gravity and the fluorescence properties of crude oils is well established, the extension of this relationship to the use of the fluorescence colours of oil inclusions as a qualitative thermal maturity guide is not justified. Fluorescence colour depends in the first instance on chemical composition, which is controlled not only by maturity but by several other processes. For example, inclusions in samples from below current or residual oil zones in the Timor Sea contain a high proportion of yellow- and orange-fluorescing oil inclusions compared to the overlying oil zones, which are dominated by blue-fluorescing oil inclusions. This observation is interpreted to be due to water washing causing molecular and gross fractionation of oils prior to trapping. Fractionation of the gross composition of oil during the inclusion trapping process may also be a significant controlling process on the fluorescence colours of oil inclusions, due to the preferential adsorption of polar compounds onto charged mineral surfaces. A trapping control is strongly supported by synthetic oil inclusion work. Care should be taken when interpreting the charge history of samples containing oil inclusions with mixed fluorescence colour populations, such as those from the Iagifu-7x well in the Papuan Basin. It is possible that the different colour populations represent a single oil charge, with oil inclusions trapped under slightly different conditions or at slightly different grain surfaces, rather than multiple migration events.

Mathematical modeling of stable carbon isotope ratios in natural gases

A new approach is presented for mathematical modeling of stable carbon isotope ratios in hydrocarbon gases based on both theoretical and experimental data. The kinetic model uses a set of parallel first-order gas generation reactions in which the relative cracking rates of isotopically substituted (k∗) and unsubstituted (k) bonds are represented by the equation k∗/k=(A f ∗/A f ) exp(−ΔEa/RT), where R is the gas constant and T is temperature. Quantum chemistry calculations have been used to estimate the entropic (A f∗/A f) and enthalpic (ΔEa) terms for homolytic bond cleavage in a variety of simple molecules. For loss of a methyl group from a short-chain n-alkane (≤ C 6), for example, we obtain an average ΔEa of 42.0 cal/mol and an average A f∗/A f of 1.021. Expressed differently, 13C-methane generation is predicted to be 2.4% (24‰) slower than 12C-methane generation (from a short-chain n-alkane) in a sedimentary basin at 200°C but only 0.7% (7‰) slower in a laboratory heating experiment at 500°C. Similar calculations carried out for homolytic bond cleavage in other molecules show that with few exceptions, ΔEa varies between 0 and 60 cal/mol and A f∗/A f between 1.00 and 1.04. Examination of this larger data set reveals: (1) a weak sigmoid relationship between ΔEa and bond dissociation energy; and (2) a strong positive correlation between ΔEa and A f∗/A f. The significance of these findings is illustrated by fitting a kinetic model to chemical and isotopic data for the generation of methane from n-octadecane under isothermal closed-system conditions. For a specific temperature history, the fitted model provides quantitative relationships among methane carbon isotope composition, total methane yield and methane generation rate which may have relevance to the cracking of oil-prone kerogens and crude oil. The observed variability of the kinetic reactivity of various methane source rocks highlights the need to apply and adequately calibrate such models with laboratory data for specific study areas. With this approach isotope data of natural gases can be used not only to estimate the time of gas generation in a sedimentary basin, but also to evaluate the source rock maturities at which specific accumulations were generated, and place constraints on trap charging histories.

Anomalous pressure generation within the Alberta Basin: implications for oil charge to the Viking Formation

Previous studies have shown that hydrodynamics played a pivotal role in the regional transport of hydrocarbons and heat in the Alberta Basin. However, the mechanisms impelling groundwater flow in the basin are controversial. Regional, topography-driven groundwater flow most likely controlled long-range oil migration and modern geothermal gradients, but anomalous pressures also existed during basin evolution. Anomalous pressures in the Alberta Basin consist of two types: overpressures in the past during sediment compaction and hydrocarbon generation, and sub-hydrostatic fluid pressures within the modern flow system. Groundwater flow, heat transfer, and hydrocarbon generation were simulated using a numerical model for a two-dimensional section of the foreland basin. Simulations are constrained using vitrinite reflectance, borehole temperature and hydrologic data. Results indicate that mechanical sedimentary loading and organic maturation produced overpressures within the Colorado Group shales and impelled oil downward to the underlying Viking Formation during the Paleocene. Low permeability aquitards (≤10−21m2) facilitate anomalous fluid pressure generation over a broad zone. The evolution of anomalous pressures within petroleum source rocks has significant implications for understanding the history of oil charge.

IDENTIFICATION AND INTERPRETATION OF LEAKING HYDROCARBONS USING SEISMIC DATA:A COMPARATIVE MONTAGE OF EXAMPLES FROM THE MAJOR FIELDS IN AUSTRALIA&amp;apos;S NORTHWEST SHELF AND GIPPSLAND BASIN

An extensive volume of 3D seismic data over a number of oil and gas fields in Australia's North West Shelf and Gippsland Basin has been examined for evidence of the effects of hydrocarbon migration and/or leakage. For comparative purposes, 2D and 3D data have also been studied over a number of adjacent traps, including dry traps and partially to completely breached accumulations. Fields and traps investigated include Bayu-Undan, Jabiru, Skua, Swift and Tahbilk in the Bonaparte Basin, Cornea in the Browse Basin, North Rankin, Chinook, Macedon, Enfield and Zeewulf in the Carnarvon Basin, and Kingfish in the Gippsland Basin. The principal goal of the study is to provide representative case studies from known fields so that, in undrilled regions, the exploration uncertainties associated with the issues of hydrocarbon charge and trap integrity might be reduced.Direct indicators of hydrocarbon migration and/or leakage are relatively rare throughout the basins studied, though the discoveries themselves characteristically show seismic anomalies attributable to hydrocarbon leakage. The nature and intensity of these hydrocarbon-related seismic effects do, however, vary dramatically between the fields. Over traps such as Skua, Swift, Tahbilk and Macedon, they are intense, whereas over others, for example Chinook and North Rankin, they are quite subtle. Hydrocarbon-related diagenetic zones (HRDZs), which had been identified previously above the reservoir zones of leaky traps within the Bonaparte Basin, have also been recognised within the Browse, Carnarvon, Otway and Gippsland Basins. HRDZs are the most common leakage indicators found and are identified easily via a combination of high seismic amplitudes through the affected zone, time pull-up and degraded stack response of underlying reflectors. In some cases (the Skua and Macedon Fields), the HRDZs actually define the extent of the accumulations at depth.Anomalous, subtle to strong, seismic amplitude anomalies are associated with the majority of the major fields within the Carnarvon Basin. The strength and location of the anomalies are related to a complex interplay between trap type (in particular four-way dip-closed versus fault dependent), top seal capacity, fault seal integrity, and charge history. In some areas within the Carnarvon, Browse and Bonaparte Basins, shallow amplitude anomalies can be related directly to gas chimneys emanating from the reservoir zone itself. In other instances, the continuous migration of gas from the reservoir has produced an assortment of pockmarks, mounds and amplitude anomalies on the present day sea floor, which all provide evidence of hydrocarbon seepage. In the Browse Basin, strong evidence has been found that many of the modern carbonate banks and reefs in the region were initially located over hydrocarbon seeps on the palaeo-seafloor.The examples and processes presented demonstrate that the analysis of hydrocarbon leakage indicators on seismic data can help to better understand exploration risk and locate subtle hydrocarbon accumulations. In mature exploration provinces, this methodology may lead to the identification of subtle accumulations previously left undetected by more conventional methods. In frontier regions, it can help to identify the presence of a viable petroleum system, typically the principal exploration uncertainty in undrilled regions.

Abstract: Implications of complex hydrocarbon charge history in Ship Shoal 274/293, Gulf of Mexico

Abstract: Implications of complex hydrocarbon charge history in Ship Shoal 274/293, Gulf of Mexico

The Sculpture of Early Medieval Rajasthan

During the early medieval period, from the seventh to the ninth centuries, the area of western India now known as Rajasthan was transformed from a politically and artistically minor region to one of relative importance. Rajasthan was the homeland of the Gurjara Pratiharas, one of the most powerful dynasties in northern India, and many important temples were produced during their brief tenure there. While these monuments provide rich sources of information about iconographic preferences and artistic styles, this book argues that they further provide important, and overlooked, clues to Rajasthan's charged early political history. In analyzing sculptural style and iconographic programs within chronological and regional parameters, the book proposes that the Gurjara Pratihara presence in Rajasthan was energizing but disruptive, particularly to dominant religious and stylistic patterns in the region.

The Victory gas field, West of Shetland

The Victory gas field is located 48 km northwest of the Shetland Isles, in UKCS Block 207/la. The structure was first drilled in 1977, the discovery well 207/1-3 encountering 224 ft (68 m) of gas-charged Lower Cretaceous sandstone at a subsea depth of 4144 ft (1263 m). No gas–liquid contact was recorded in the well. Five wells have been drilled on the Victory trend to date with the most recent, 207/la-5, being drilled in 1996. The Victory Field was originally mapped with 2D seismic data as a structural high associated with a rotated fault block. In 1996, 3D seismic data were acquired over the area as part of a Shell-operated group-shoot and the improvements in resolution and data quality have prompted a re-evaluation of the field. The 3D seismic data have demonstrated that the Rona Ridge and the linked West Shetland Basin in this area have undergone a complex geological history with multiple phases of Cretaceous tectonism. The structural elements that affect trap configuration and reservoir sand distribution are related to faulting which follows both a NE–SW Caledonian trend and an offsetting NW to SE fault trend. The reservoir is of Early Cretaceous (Late Aptian–Late Albian) age and comprises shallow to marginal marine sandstones and conglomerates overlying Precambrian basement. Reservoir quality is consistently high. Geochemical analyses of both gas and oils encountered within the Victory field area have been undertaken and tied both to basin-scale modelling of the petroleum system and structural mapping at a field-scale. These studies demonstrate the complex, multi-phase charge and alteration history of the trapped hydrocarbons of the Victory Field.

The Paleocene deepwater sandstone play West of Shetland

Following the discovery of oil in Paleocene sandstones in the Foinaven and Schiehallion fields in 1992 and 1993, exploration in the Faeroe–Shetland Basin has been very active. Most of the basin has been covered with 3D seismic surveys and numerous wells have been drilled for the Paleocene deep water play. However, success in terms of commercial discoveries has been limited. Detailed analysis suggests that the Foinaven/Schiehallion geological setting is unique in terms of hydrocarbon charge history, reservoir quality and trapping style. Oil charge is attributed to the presence of a Mesozoic high (Westray Ridge), which both focused charge and retained oil prior to remigration into Paleocene traps. The T35–T36 sequence which includes the Andrew Tuff appears to provide an effective top seal across the basin and all significant discoveries have been made in the T31–T35 fairway directly underlying it. Although reservoir sandstones similar to those found in the Foinaven and Schiehallion area have been encountered across the entire Faeroe–Shetland Basin, their relatively shallow burial depth and their position at the base of slope at Foinaven and Schiehallion appears to be unique in the basin. Several gas discoveries have been made in the Flett Sub-basin, located to the northeast of the Foinaven/Schiehallion area. The Flett Sub-basin has all the ingredients of a successful Paleocene hydrocarbon play, such as pervasive charge, an effective regional top seal, and stacks of Lower Paleocene deep-water sandstones and shales. However, trap definition is difficult: most remaining structures rely on a stratigraphic trapping component. The definition of top reservoir sand on seismic data is not straightforward, as seismic imaging techniques, including direct hydrocarbon indicator (DHI) analysis, are often ambiguous. T31–T35 sandstones in the deep Flett Sub-basin tend to be tight, although a considerable number have been found to have good reservoir properties. Prediction of areas where reservoir quality is preserved will be a main challenge for continuing exploration in the deep part of the Faeroe–Shetland Basin, including the undesignated area adjacent to Quadrant 204.

A study of the gross compositions of oil-bearing fluid inclusions using high performance liquid chromatography

A high performance liquid chromatography (HPLC) method for assessing the compositions of crude oils and fluid inclusion oils is described. The necessary increase in instrumentation sensitivity was accomplished using a narrow bore Waters Spherisorb column with 5 μm particle diameter. This HPLC method has advantage over the Iatroscan TLC–FID technique previously described as the <C 16 fraction is measured by the HPLC method. However, due to uncertainties in the calibration procedure, the HPLC method is only semi-quantitative. Aliphatic hydrocarbons were calibrated taking into account the molecular weight distribution of a typical oil derived by gas chromatography. Aromatic hydrocarbons were calibrated using standards according to retention times, but the response factors span two orders of magnitude, dependent on ring size. Polar compound response factors are strongly standard dependent. The gross composition of a suite of 12 inclusion oils and 17 co-occurring crude oils from various basins in Australia and Papua New Guinea have been analysed. The inclusion oils at the Jabiru and Tirrawarra wells are enriched in polar compounds relative to the co-occurring crude oils from these wells. This is attributed to an adsorption effect during trapping. However, many inclusion oils do not appear to be polar enriched, which may relate to trapping of inclusion oils in sealed fractures as well as on overgrowths. Inclusion oils tend to be relatively depleted in aromatic hydrocarbons, which could reflect a trapping phenomenon, with preferentially less aromatic hydrocarbons and/or more waxy aliphatic hydrocarbons being trapped in fluid inclusions. Alternatively, this may be a characteristic of the different chemistry of oils that are preserved early in the charge history of a reservoir.

Constraining the oil charge history of the South Pepper oilfield from the analysis of oil-bearing fluid inclusions

The South Pepper oilfield, located in the Barrow Sub-basin on the NW margin of the Australian continent, has experienced a multi-phase charge history. Abundant oil-bearing fluid inclusions are present in samples from within the current gas cap, suggesting that an oil column existed prior to gas. This palaeo-oil (gas-leg FI oil) has Ts/Tm and C 29/C 30 αβ hopane ratios of ∼1 and the C 35 homohopanes are a significant proportion of the extended homohopanes. It has lower Pr/Ph and diasterane/sterane ratios than the currently reservoired live oil and contains gammacerane, a series of peaks tentatively identified as C 30 to C 34 17α(H)-30-norhopanes and a large amount of 2α-methylhopanes. Collectively, geochemical analysis of the gas-leg FI oil suggests that it was generated from a less mature, more calcareous source rock, deposited under more reducing conditions than the Upper Jurassic Dingo Claystone, the main source of the live oil. In addition the presence of C 30 dinosteranes in the gas-leg FI oil provides a Triassic or younger age constraint. This makes Palaeozoic carbonates an unlikely source. Possible source intervals for the gas-leg FI oil are thin, Lower Jurassic limestones and marls which occur at the base of the Lower Dingo Claystone, or a thin limestone unit (the Cunaloo Member) at the base of the Triassic Locker Shale. Samples from within the present oil-leg also contain abundant oil inclusions, consistent with high oil saturations at the present day. However, these oil inclusions exhibit different fluorescence colours, suggesting they represent a second oil charge. Geochemically the oil-leg FI oil has an intermediate composition between the live oil and the gas-leg FI oil, suggesting that gas charge displaced the first oil charge, samples of which were preserved as fluid inclusions in the oil-leg sample. Biodegradation of the first oil charge, indicated by the presence of 17α(H)-25-norhopanes in the currently reservoired live oils, can be attributed to the ingress of meteoric waters, probably during sub-aerial exposure of the basin margin during Miocene wrenching. Changing environmental conditions curtailed bacterial activity and allowed unaltered oil sourced from the Dingo Claystone to accumulate below the gas cap and mix with the biodegraded residues of the first oil charge to achieve the live oil composition. The biodegradation event must have preceded the second charge as the live oil contains compounds such as n-alkanes which would have been removed had alteration occurred after the second charge. Complex charge histories are common and the analysis of palaeo-oils trapped within fluid inclusions provides the opportunity to achieve a more comprehensive assessment of hydrocarbon charge

History of hydrocarbon charge on the Atlantic margin: Evidence from fluid-inclusion studies, West of Shetland

Fluid inclusions in Jurassic reservoir sandstones help us to understand the history of hydrocarbon charge in the West of Shetland region. The sandstones contain both hydrocarbon and aqueous fluid inclusions. Hydrocarbons are present in trails of secondary inclusions through detrital grains, and as primary inclusions in quartz overgrowths. This is direct evidence for at least two episodes of hydrocarbon charge before the present oil entered the rock, consistent with indirect evidence from burial history modeling and organic geochemistry. Fluorescence colors indicate that the two oils are of distinct composition. The first oil was degraded, reflecting an episode of uplift and exposure to near-surface fluids before reburial and migration of a second live oil. Oils comparable to these two paleocharges were mixed to produce the oils that are encountered in the current reservoir pores, showing that they were preserved despite repeated tectonic events at the Atlantic margin.

Unravelling complex filling histories by constraining the timing of events which modify oil fields after initial charge

Abstract Complex and multiphase charge histories are a feature of many hydrocarbon discoveries. Previous descriptions of charge history that have relied on the chemical properties of hydrocarbons do not define the geometry of hydrocarbon accumulations prior to the attainment of their present state. In this paper, examples of oil charge studies conducted on hydrocarbon discoveries from the Australian North West Shelf are presented to demonstrate the application of a new technique for the mapping of hydrocarbon charge called GOI™ (Grains containing Oil-bearing fluid Inclusions). In Australian oil fields GOI values in oil leg samples are an order of magnitude higher than in underlying water zones and record the maximum oil saturation experienced through time. An empirical threshold for oil saturation consistent with accumulation (GOI &gt; 5%) rather than migration (GOI &lt; 1%) of oil has been established from a database of 20 Australian oil fields. Oil inclusions are retained if oil is lost from the pore spaces of the rock, which allows GOI to be used to identify relict oil columns and locate the original oil-water contact. GOI measurements allow the original size and disposition of palaeo-oil columns to be determined and the physical events controlling the composition and size of hydrocarbon accumulations to be deduced in space and time. These data allow issues which cause changes to the original fluid contacts, such as trap integrity, tilting and gas displacement, to be confidently identified and characterized. When combined with conventional approaches to prospect evaluation, these new data allow a more sophisticated description of the filling history of hydrocarbon discoveries. They also allow the oil charge risk associated with new plays to be appropriately constrained before new drilling is commissioned.

Patterns of current and lifetime substance use in schizophrenia.

A structured interview and standardized rating scales were used to assess a sample of 194 outpatients with schizophrenia in a regional Australian mental health service for substance use, abuse, and dependence. Case manager assessments and urine drug screens were also used to determine substance use. Additional measurements included demographic information, history of criminal charges, symptom self-reports, personal hopefulness, and social support. The sample was predominantly male and showed relative instability in accommodations, and almost half had a history of criminal offenses, most frequently drug or alcohol related. The 6-month and lifetime prevalence of substance abuse or dependence was 26.8 and 59.8 percent, respectively, with alcohol, cannabis, and amphetamines being the most commonly abused substances. Current users of alcohol comprised 77.3 percent and current users of other nonprescribed substances (excluding tobacco and caffeine) comprised 29.9 percent of the sample. Rates of tobacco and caffeine consumption were high. There was a moderate degree of concordance between case manager determinations of a substance-use problem and research diagnoses. Subjects with current or lifetime diagnoses of substance abuse/dependence were predominantly young, single males with higher rates of criminal charges; however, there was no evidence of increased rates of suicide attempts, hospital admissions, or daily doses of antipsychotic drugs in these groups compared with subjects with no past or current diagnosis of substance abuse or dependence. Subjects with a current diagnosis of substance use were younger at first treatment and currently more symptomatic than those with no past or current substance use diagnosis. The picture emerging from this study replicates the high rate of substance abuse in persons with schizophrenia reported in North American studies but differs from the latter in finding a slightly different pattern of substances abused (i.e., absence of cocaine), reflecting relative differences in the availability of certain drugs.

Modeling the Galileo dust measurements at Jupiter

We discuss our modeling that is aimed to reproduce the observed dust impact rates during the first two Ganymede encounters of Galileo. We use a detailed fields and particles model of the magnetosphere and simultaneously follow the dynamics and the charging history of small dust particles. We assume Io to be the only source of the escaping dust grains and show that our model matches the gross characteristics of the observations. We argue that the large scale dawn‐to‐dusk electric field can be responsible for shaping the emission pattern of dust from the Io plasma torus. The emission pattern shows a minimum flux toward the Sun, perhaps explaining why the observed fluxes did not increase as Galileo approached the source region of the dust particles. We show that the general shape of the trajectories of the grains offers a good explanation for the sudden shift in the observed direction of the dust impacts. Modeling the observed dust impact rates gives an excellent opportunity to test our particles and fields model of Jupiter's magnetosphere.

Charge dynamics of a methane adsorption storage system: Intraparticle diffusional effects

The charge of natural gas adsorption storage systems is studied numerically, With emphasis given to the impact on its dynamics of intraparticle diffusional resistances to mass transport. Besides adsorption kinetics and thermal effects, the simulation model takes into account both mass transport inside the adsorbent and hydrodynamics of flow through the packed bed. Numerical results are presented for change with methane of a 50 liter cylindrical reservoir, filled with hypothetical adsorbents with diffusional time constants in the range 10−3 s1D/Rp2 ≤ ∞. and with the adsorption equilibrium curve of a commercially available activated carbon with a good adsorptive storage capacity. An attempt is made to assemble the charge histories for different values ofD/Rp2 , in a single cure by using a modilied time scale.

Australian Petroleum Geoscience Research: Its Influence on Exploration and Production in Australasia: ABSTRACT

Petroleum geoscience research in support of efficient and effective exploration in Australia and focused on the reduction of exploration risk has never been healthier. The recognition and mapping of new oil families, new methods to predict trap integrity and charge history, new techniques for fluid history analysis, the development of new explanations and models for the development of large portions of the Australian margin and the development of information systems that meet the needs of explorers are reducing risk. The scale of the research undertaken and the degree of collaboration between industry and research groups is remarkable. There is a productive balance between groups developing and applying new technology and those undertaking regional geological and petroleum systems research. The balance has occurred because of the long term commitment by the Australian government, via legislation and funding, to ensure the preservation of exploration data in national geoscience database systems and that basic and applied research at all scales from basins to wells is undertaken in support of petroleum exploration and development. The reduction in the number of geoscientists in major companies and the number of major companies undertaking exploration in Australia has resulted in a shift in tasks traditionally undertaken bymore » these companies to consultants, contractors, universities, and government agencies. The scale and complexity of the questions that need to be solved has forced significant and productive collaborations.« less

Native American Families and Child Neglect

This paper presents findings from an NCCAN-funded study of family functioning in neglecting families and in a comparison group of low-income families with no prior referrals for child neglect. It examines a sub-sample of a larger study of 77 Native American families from two states: Iowa and Oregon. Data were gathered through structured interviews with families and self-report measures completed by the primary caregiver and the children. Two—way analysis of variance was used to test differences between the neglect and comparison groups, controlling for per capita income and tribe. This study found that the family's current situation was more important than the caregiver's history in differentiating neglecting from comparison families. Multiple problems, substance abuse, and a history of criminal charges were strongly related to neglect. The number of fathers in the family was also related to the risk of neglect and regular contact with a father was the only type of social support that reduced the likelihood of neglect. Since this study relied on non-probability sampling, the findings are not generalizable to other populations. However, the results confirm previous research and suggest hypotheses for further testing.

Fault Seal Behavior at Beryl Field, UK North Sea: Observations from 20 Years of Production, Drilling and Injection Data: ABSTRACT

The Mobil operated Beryl Field is located in UK Block 9/13, on the west flank of the South Viking Graben. Hydrocarbons are produced from five Jurassic and Triassic sandstone reservoirs within several large, faulted-bounded structural traps. Within Beryl Field, extensive data, including 150 wells, and a 20 year production history provides an excellent opportunity to study fault-fluid interactions in the subsurface. Individual fault compartments within the field exhibit a complex charge and seal history with temporal variations over both geologic and production time scales. Sealing faults play an important role in subsurface fluid distribution within the field as demonstrated by different oil-water contacts (up to 1000 ft), distinct pressure cells (up to 2000 psi differential) and unique geochemical and PVT parameters observed between adjacent fault-bounded compartments. Both membrane (sand-on-sand) and juxtaposition (sand-to-shale) fault seals are interpreted in the field. Based upon cored fault rocks, clay smear deformation is the primary mechanism responsible for the seal potential of the membrane seal fault zones. Broad deformation zones comprised of clay smear and dense cataclastic shear bands typically surround larger faults and locally minor diagenesis is observed. The history of hydrocarbon production and gas/water injection in the field has highlighted the temporal variabilitymore » of fault seal behavior within the field. Examples of faults that sealed over geologic time but which broke-down during production are observed. Faults control fluid movement within the reservoir and degrade reservoir productivity adjacent to fault damage zones. Incorporation of fault transmissibilities and seal potentials into reservoir simulation models is necessary to evaluate reservoir and fault behavior under changing production conditions. An understanding of how faults influence reservoir behavior through production time is critical to the success of the field development plan.« less

LATE TERTIARY FLUID MIGRATION IN THE TIMOR SEA: A KEY CONTROL ON THERMAL AND DIAGENETIC HISTORIES?

An integrated study of the fluid flow, hydrocarbon charge and thermal histories of a suite of traps from the Timor Sea has revealed the presence of a key fluid flow event in the latest Miocene/Early Pliocene. This event, which coincided with, and was driven by, the extensive tectonism and fault reactivation associated with the collision of the Australian and Eurasian plates, involved the flowage of hot (90-120°C), saline (&gt;200,000 ppm) brines (probably from deeply buried Palaeozoic evaporite sequences) up major faults and through the Mesozoic and Tertiary sequences. The passage of these hot brines produced a Late Tertiary (100,000 to 1 million years). In contrast, strongly reactivated, low integrity traps such as East Swan appear to have been breached too quickly (

Hydrocarbon-Related Diagenesis: The Key to Understanding Trap Integrity and Hydrocarbon Charge History in Australia's Timor Sea?: ABSTRACT

In the Late Miocene, the effects of continental collision partially to completely breached many charged, Mesozoic traps in Australia`s Timor Sea. An integrated geophysical and geochemical study has shown that the biodegradation of hydrocarbons leaking from these Mesozoic traps produced hydrocarbon-related diagenetic zones or HRDZs within overlying, Eocene aquifer sands. These HRDZs, which are principally defined by zones of intense carbonate cementation, are seismically-resolvable, with both their size and velocity being directly related to the amount of hydrocarbons that have passed through the aquifer sands. Consequently, the integration of observations on the seismic, velocity and isotopic characteristics of the HRDZs, the nature of present-day hydrocarbon seepage (as defined by sniffer and ALF data), and geometric relationships between the reactivation (Miocene) and trap (Jurassic) fault geometries, can provide a powerful predictive tool for evaluating and ranking undrilled structures and prospects within the Timor Sea.