Oil and gas exploration is an investment and the aim is to get a profitable return.However, there are many aspects that introduce uncertainty and therefore increase the risk oflosing money. In order to control the return, JNOC started quantitative geologic risk evaluationand ENPV (expected net present value) evaluation in JNOC's investment projects. JNOC evaluates a prospect in “geologic chance of success (probability of finding oil or gas)”and “probabilistic reserve distribution (if oil or gas is found, what volume is recoverable)”. Thereserve distribution is changed to monetary distribution by estimating development andproduction scenarios. The situation is categorized into three categories, economic success (getprofit), geologic success but economic failure (lost money), and geologic failure (lost money).ENPV is one of the expected value, the summation of the products of chance and monetary valueof each situation. ENPV is a tool to find the appropriate "entrance fee" to a project, to characterizeand compare projects regarding riskreturn and to make a portfolio. It is important to improve evaluation accuracy to better control returns. Therefore we also doa reality check of each evaluation and post audit systematically.
Geochemical studies on formation waters collected from oil and gas fields in Akita and Yamagata Prefectures, northeastern Japan, were carried out in order to clarify chemical characteristics, origin and alteration processes of them. The formation waters are chemically characterized by depleted Cl-, SO42- and Mg2+, and enriched HCO3- I-, HBO32- and NH4+ compared with the sea water. They have a relatively narrow range of δD values between -10 and +1‰ and a large variation of δ18O values from -3.6 to +2.5‰, which are similar to formation waters in Niigata. The measured Sr2+ contents of them range from 4.1 to 51.8 mg/l. The formation waters have Sr isotopic values ranging from 0.70622 to 0.70764, which are less than those of the sea waters from Miocene. These chemical and isotopic data show that the formation waters were originated from the sea water entrapped during deposition and subsequently altered due to the interaction with rocks, organic matters and so on.
Magnetic properties of core samples in stratigraphic test wells are presented in an attempt to apply to comprehensive geological interpretation. Stability parameter of magnetization of igneous basement cores in MITI “Kesennuma-Oki” and “Goto-nada” indicates that rock magnetic property is essential for reliable interpretation of geomagnetic anomaly in sedimentary basins. Sedimentary core samples from MITI “Kesennuma-Oki”, “Soma-Oki”, “Joban-Oki” are described to obtain information on core-orientation. Magnetic cleaning for them shows that drilling-induced secondary magnetization is common in deep borehole samples. After careful rock magnetic cleaning, a site-mean direction of primary remanent magnetization for the Cretaceous sample in MITI “Soma-Oki” has large westerly declination. Together with other paleomagnetic data, the present result suggests a differential tectonic movement and reorganization of sedimentary basins in northeast Japan since the Cretaceous. Thus rock magnetic study of borehole samples contributes to understand the origin of basement, tectonic history, and regional basin evolution.
For an extended reach drilling simulation, many important techniques arerequired. Torque and drag prediction is one of them. There is an opinion that catenary trajectorycan minimize drag. However, as drag is accumulated upward, drag is affected not only by bendingrate but by hook load and inclination angle at lower end of catenary. For a temporary evaluationwhether drag of catenary is the minimum or not, two types of trajectory are compared throughsimulation study. One is an approximate catenary trajectory which composed of multiple build upsection with different bending rate, and another is equivalent build up section which has the sameinclination angle and lower end as the former. The following findings are obtained. (1) Drag of approximate catenary trajectory does not remarkably decrease comparing with thatof equivalent build up section. (2) When drag of catenary trajectory decreases comparing original build up section, the majordecrease of drag may be contributed to the connecting downward tangent section. (3) In this sense, the effect of catenary for drag decrease may be said that catenary can increaseinclination angle, since high inclination angle can decrease drag over downward tangentsection.
For a design of deviated well trajectory, it is important to estimate torque and drag and to select the most appropriate well trajectory. Some equations of torque and drag have been proposed under an assumption that torque and drag forces are primarily caused by sliding friction. However, since they are formulated as an incremental equation using short element of the pipe, they have a little short versatility, i.e., necessity of computer usage. Under an assumption that azimuth angle and inclination angle do not change together over a bending section, the incremental equation can be transformed to differential equations. Based on conducted differential equations for three bending sections, theoretical formulas for inclination angle change section and approximate formula for azimuth angle change section are both derived as algebraic equation. These formulas can easily calculate torque and drag using only bending condition parameters. It is found that the proposed formulas can be used as a convenient estimating method, since the estimate for hookload including drag agrees well with one obtained by conventional iteration of the incremental equation.
The Tertiary movements of the Indo-Australian plate are re-evaluated, based on the magnetic anomaly zones on the south off the Australian continent. The re-evaluated history of the plate's movements shows a good consistency with the Tertiary tectono-stratigraphic records of tectonic and sedimentary events in petroliferous West Indonesia. The magnetic anomaly zones studied are distributed in the area between the George V Fracture Zone and the Australia-Antarctica Magnetic Discordance Zone. The magnetic anomalies are symmetrically developed with respect to the Southeast Indian Oceanic Ridge in the area. They preserve laterally and chronologically high continuity. The re-evaluation indicates more detailed historical changes in the northward movement of the Indo-Australian plate during the Tertiary than the previous works. This study shows two geohistorical phases in the Indo-Australian plate movements: (I) a slow northward movement of the Australian plate till the latest Eocene, and a sudden acceleration of the movement around the earliest Oligocene, (II) the Late Oligocene acceleration and a plateau of high movement rate till the late Early Miocene, and the early Middle Miocene remarkable acceleration. These events in the plate movements were well recorded in the mega-sequence of the Indonesian Tertiary and Quaternary systems as well as stratigraphic controls of the eustatic sea level fluctuations. The tectono-stratigraphic history in the West Indonesia established an excellent petroleum systems that have been evaluated to yield ca. 46.5 billion barrels of oil equivalent of recoverable hydrocarbons in total.
Since the finding of the first oil in 1953, many wells have been drilled in the North West Shelf (NWS). From 1960's to 1970's, there were significant discoveries of oil/gas fields in Carnarvon and Browse Basins, which were the anticline trap based on the 2D seismic interpretation. In the early 1980's, the new oil fields found in the Bonaparte Basin showed that these oil fields were not only anticline trap but also fault trap, which were based on the regional geological and geophysical studies. In the early 1990's, the improvement of seismic technology led to the finding of additional reserves, which were staratigraphic trap and other type traps. In 1998, sixty-one exploration and appraisal wells were drilled on the NWS. The Carnarvon basin had the only significant oil and gas fields in 1998. In early 1999, every company decided to reduce the exploration budget under the influence of the low oil price ($10/bbl). As a result, these exploration companies take the exploration strategy to concentrate on the core area for each company. This strategy will lead to the increasing exploration activity on the exploration mature basin, as the Carnarvon Basin.