Migration and accumulation of hydrocarbons are part of processes in petroleum system in the past, so the processes including generation and expulsion of hydrocarbon from source rocks are invisible at present. Analysis of compaction curves conducted by Dr.Magara in 1966 was the first proposal on migration and accumulation of petroleum quantitatively and his idea led to the quantitative discussions in petroleum exploration industry in Japan. Dynamic fluid model based on muti-phase Darcy flow had been developed and accepted at first in the world. Recently, various kinds of methods for basin simulation are developed based on static model, such as Ray Tracing (Flowpath) and Invasion Percolation methods. Furthermore, hybrid model is the latest model, which can be used as both of dynamic and static methods in case by case at the same time. In this paper, we consider the analysis of compaction curves shown by Magara (1966a) as the first breakthrough of quantitative analysis for fluid migration. We introduce the historical transition of quantitative basin model since Magara's idea of fluid migration had been proposed. Fluid migration models especially based on static model are introduced, and future orientation of quantitative analysis in petroleum exploration is indicated as personal view of authors. This paper is dedicated to Dr. Magara who tried to understand the compaction process quantitatively as the pioneer and gave us the first step of petroleum system analysis, such as sedimentary basin model.
An oil (or gas)/water contact in an oil and gas field is normally horizontal because of a gravitational control in a petroleum trapping mechanism. Inclined contacts, however, have been observed in a significant number of fields. The Cushing gas-oil field discovered in 1912 in the Central Plateau, Oklahoma, USA, is the first example of the field with the inclined contacts. After the Cushing discovery, many fields of inclined contacts have been discovered in the Great Prairie of USA. The inclined contacts of these fields are caused by gradients of hydraulic heads ; the gradient is caused by a difference between an altitude of mountainous areas (recharge areas for aquifers) and a depth of subsurface aquifers (reservoir at a field in the prairies). Since 1990, oil and gas fields with the inclined contacts have been found in marine environments. The Peciko gas field is the example from the Mahakam Delta Province, Indonesia. A number of oil and gas fields with the inclined contacts have been discovered in the North Viking Basin. The marine type of inclined contacts should be considered to be controlled by lateral gradient of pore-pressures. The gradient is caused by a lateral transfer of overpressures from neighboring, stratigraphically-underlying, overpressured formations. Hubbert (1953) proposed hydrodynamic mechanism of entrapment of oil and gas fields associated with inclined contacts. The theory has been the most commonly accepted theory for inclined contacts ; a flow of formation fluids hydrodynamically causes a gradient of hydraulic heads that inclines the contacts. Hydrodynamic effect on the gradient, however, should be estimated to be negligible based on a series of model calculations of the velocity heads and friction loss heads. It is because a rate of subsurface flow through aquifer (reservoir) is too small to cause a sufficient magnitude of hydrodynamic effect.
Chemical osmosis is a phenomenon which is caused by the behavior of clay layers and mudstones as semipermeable membrane. It is considered that the osmotic behavior can be important for the generation and preservation of abnormal pore pressures in subsurface in the case where solute concentration is different between both sides of the mudstone layer. Abnormal pressures caused by the chemical osmosis are dissipated by diffusion-limited process, and hence, the abnormal pressures can be preserved in geologic time scales. This lecture presents examples of the chemical osmotic processes inferred and/or observed in subsurface environments. Our effort to analyze fundamental processes related to chemical osmosis is also presented. An example result from our experiment clearly shows that the mudstone behaves as semi-permeable membrane. Further study to determine physical properties of mudstones which are related to chemical osmosis is necessary by laboratory experiments presented in this lecture.
High resolution 3D seismic survey “Tokai-oki to Kumano-nada” was conducted for methane hydrate exploration in the eastern Nankai Trough by METI in 2002. Our study focuses on zigzag-shaped specific reflectors which occur on BSR margins on the 3D data. We call the reflectors “Foldback Reflectors (FBRs)” in this study. From the edge of BSR, the 1st FBR generally extends down to the lower formation below the BSR and cross sedimentary horizons. The following FBRs extend down from the edge of the upper FBR forming accordion-like shape. The 1st FBR indicates normal polarity, and the following FBRs change their polarities alternately. FBRs are mostly developed in the well stratified formation but not in the area showing frequent fractures and major lateral lithological change. Dip direction of each FBR is probably controlled by that of crossing formations. FBR generally corresponds to lateral seismic facies boundaries between the BSR distribution area and the area outside the BSR distribution. The formation beneath the BSR shows dimmed seismic facies characterized by relatively low amplitude and lack of high frequency components with relatively low velocity in contrast to the area outside the BSR with normal seismic facies. The lowest FBR does not cross over major unconformities, which often exhibit negative polarity suggesting fluid bearing strata. In addition, high amplitude layers are sometimes described at foldbacks convex to the area outside the BSR. These high amplitude layers probably having higher permeability are interpreted as conduits for outward migration of gas-related fluids which are distributed in the low velocity area. The shapes of FBRs are possibly related to layer-parallel migration in strata with wide range of permeability. From these observations, FBR can be regarded as an important proxy indicating migration front of gas-related fluid.
Many fields reportedly have tilted oil water contacts (OWC). In the target carbonate reservoir of this paper, deeper OWCs, higher oil pressures, deeper free water levels (FWL) and larger transition zones were observed towards the direction of recent structure tilting while no difference in pressure was observed in the aquifer. From these observations and structure restoration and geochemical studies, it was considered that recent structure tilting after oil accumulation induced oil remigration and influenced the current fluid and pressure distributions of this reservoir. This idea was supported by the numerical experiments in which effects of capillary pressure hysteresis were considered.
The petroleum system in the deep water of the Toyama Trough to the southwest offshore of the Sado Island in the Japan Sea was re-evaluated based on the results of newly-conducted basin modeling simulation, fluid inclusion analyses of METI Sado Nanseioki wells, and interpretation of METI Sado Seiho 3D seismic survey. METI Sado Nanseioki wells demonstrated the distribution of thick Neogene sediments in the deepwater region and confirmed a 15 meter-thick oil column in the lower part of the Shiiya Formation at METI Sado Nanseioki S. The middle Miocene Lower Teradomari Formation is thought to be a source rock of the oil. This source rock is considered to contain a mixture of Type II and III kerogens and to have good source rock potential (0.90-2.98%TOC). The bitumens include both marine and terrestrial organic matter, notably characterized by very abundant oleanane. The GCMS analysis indicates that hydrocarbon inclusions in the Lower Teradomari Formation have the same characteristics as the oil recovered from Shiiya Formation. It implies that the hydrocarbon was temporally trapped in the Lower Teradomari Formation, and then migrated to the overlying Shiiya Formation reservoirs through faults in a very short space of time. The basin modeling simulation results indicate the following interpretations ; · Hydrocarbon generation has started from the late Miocene in the depression to the southwest of AWABI structure. · Hydrocarbon is currently being generated in the depression around the AWABI and BURI structures. · Hydrocarbon has migrated through the D location of METI Sado Nanseioki since the Late Miocene. The re-evaluation results provide deeper understanding of the petroleum system in the area and suggest that structural traps at the Lower Teradomari Formation level located in the surrounding area would be favorable for efficient accumulation of hydrocarbons at the present day.
Deepwater area in the Southern Gulf of Mexico is one of the most attractive areas for future petroleum exploration. JOGMEC carried out a petroleum system evaluation study jointly with PEMEX-E&P in this region using existing wells and seismic data acquired by PEMEX-E&P. The study consists of 1) 1D thermal modeling at the well locations to obtain heat flow variation, 2) map-based modeling to identify effective kitchen areas and migration paths for the hydrocarbon generated in the source rocks, 3) tectonic restoration on the 2D seismic sections along the migration paths, and 4) 2D petroleum system modeling to understand hydrocarbon charge history in the study area. Due to complicated geology with salt intrusions and thrusts, conventional modeling workflow is not valid to fully reveal the hydrocarbon charge processes in the study area. In this paper, authors briefly introduce a suitable modeling workflow and the methodologies which we applied to the study.
Molecular and isotope compositions of gas samples from oil and gas fields, and two exploration wells in the Kitakanbara area were analyzed. Most gases are classified into mixture of microbial and thermogenic gases. Maturity assessments of the reservoir gases were performed using ethane and propane carbon isotope compositions in two representative fields in the area, the Iwafuneoki and Higashiniigata fields. In both fields, vertical migration of thermogenic gas through a large fault system is inferred. In one exploration well, thermogenic gas migration to reservoir horizons was observed based on the carbon isotope composition of headspace gas, mud gas and drill stem test gas samples. On the other hand, no migration of thermogenic gas was detected in the other exploration well, despite hydrocarbons are deposited in the Higashiniigata field located in the updip of the well, suggesting that hydrocarbons passed through a deeper level below the total depth of the well. These results suggest that the hydrocarbons generally migrated laterally within the source horizons and did not migrate vertically into shallower reservoir horizons until the hydrocarbons reached the large fault systems.
Relating to the assessment of the cap rock integrity on CO2 geological sequestration, we measured threshold pressure for supercritical CO2 under pressure and temperature conditions of 1,000 m depth (i.e., 10 MPa and 40°C). The present study aims to quantify various factors which affect the variability of rock's threshold pressure and to model the threshold pressure range of a cap rock. We prepared sintered compacts composed of uniformly-sized silica glass beads (0.2-10 μm), and examined a correlation between threshold pressure and permeability for each diameter. The result revealed that threshold pressure of sintered compacts increased drastically with a decrease of component particle size. Moreover, there existed a good linearity between the logarithm of threshold pressure and the logarithm of permeability. The fitted line is an important basis specifically from a viewpoint of safety assessment because it corresponds to the standard value of threshold pressure of homogeneous rocks. For threshold pressures of samples composed of smaller size particles, measured values were much lower than theoretical values. Although it is required to verify the discrepancy from an ideal particulate alignment of our samples and the validity of used contact angle value, the present result suggests that supercritical CO2 may have a potential to permeate a cap rock easily more than expected.
Development of Preferential Corrosion of Weldment (PCW) was found in natural gas gathering lines in a sweet gas-condensate field. Effects of inhibition on these unique corrosion phenomena were studied. Effects of Cr and/or Mo addition to weld metal were also examined. At least 500 ppm of the inhibitor now in use is required to properly control the corrosion. The section of selective corrosion varied with inhibitor concentration. 0.5% addition of Mo effectively controls the selective localized corrosion of the weld metal. Although Cr is a beneficial alloying element to mitigate CO2 corrosion, 2%Cr-1%Mo weld metal selectively corrodes with concentration of the inhibitor 250 ppm. Galvanic effects on this preferential corrosion were not confirmed.
Electromagnetic (EM) techniques for oil and gas exploration have been used for many years. These methods include magnetotelluric (MT), marine magnetotelluric (MMT) and during the last few years, marine controlled source electromagnetic (CSEM). The basic idea behind the use of marine CSEM for hydrocarbons is to identify resistive layers in an otherwise conductive environment. The fundamental equipment requires to conduct a marine CSEM survey consists of seafloor EM receivers and an electric dipole transmitter. A horizontal electrical dipole is towed close to the seafloor inducing a low frequency. EM signal could be recorded by stationary sea bottom receivers.