The problem of exploration petroleum

The logistics problems of supporting exploration and exploratory drilling in remote locations of the world have been well publicized recently. The problem being recognized more frequently and requiring particular attention during development of the Prudhoe Bay field since 1975 is that remote-location logistics do not end with exploration and drilling. Installation of production facilities poses problems that are just as production facilities poses problems that are just as intricate, potentially expensive, and difficult to solve. This has occurred offshore as discoveries are made in deeper and rougher water; the frontier arctic areas also are prime examples of where logistics problems are being encountered onshore.

Petroleum geology is general geology with a specific aim, and all thesethings apply to petroleum geology. The petroleum geologist’s work also has its descriptive and interpretive aspects, but the emphasis tends to linger on the descriptive because the goal of petroleum geology is a deterministic model of the area under study — ultimately, the oil or gas field. To achieve this goal, the specialists of petroleum geology tend to work in teams (whichalso broadens their minds).Petroleum geologists, whatever their speciality, tend to become either

exploration geologists or development geologists.

The difference is not only a matter of scale, but also of outlook that can be so different that there is danger of the one not understanding the other properly. Petroleum exploration, in its simplest terms, consists of studying large regions that do or could contain petroleum, identifying progressively smaller areas of progressivelygreater interest in these until a prospect worth drilling has been identified, and discovering oil or gas in one or more of these. The development geologist starts with the discovery well and a detailed seismic survey, and locates appraisal wells to assess the size and nature of the accumulation or accumulations.If petroleum is found to be in commercially viable quantities, the development geologist seeks to obtain an accurate model of the accumulation on maps and cross-sections that can be used for estimating the able reserves and the siting of development wells that will produce these reserves as efficiently as possible.

The exploration geologist is concerned with regional geology deduced from surface outcrop, geophysical surveys, and the results of any boreholes drilled in the area. In spite of the enormous advances in geophysical techniques, the stratigraphy of the area may only be determinable in a rather general sense. The development geologist, on the other hand, is concerned with the detailed stratigraphic sequence, and its structure, over a relatively small area. Yet this detailed stratigraphic sequence is not obtained from a study of the rocks themselves, but rather from the electrical and other geophysical responses obtained in the boreholes.

It is commonly said that the petroleum geologist has the advantage of working without financial restraint. This is rarely, if ever, true. It is true that many areas are investigated using several disciplines, some of which (geophysics, drilling) are very expensive to apply, but all will be operating under some financial restraint. It is not so often said, but is nevertheless true, that most petroleum geological work has a time constraint (which is also a financial constraint) and so the conclusions may be based on inadequate data. Much of this work is carried out in parts of the world that would otherwise have waited decades for investigation, and it is carried out with

time limitations. The danger of false inference is always present. When such work finds its way into print, the conclusions are likely to be accepted by geologists with no local knowledge. There are also competitive restraints. It would be invidious to mention specific published articles, but we have all experienced the frustration of papers that give formation names but no lithologies, the stratigraphy of the petroleum-bearing part of the sequence but not of those overlying and underlying it.

The study of petroleum geology centreslogically around the three main processes — petroleum generation, migration, and accumulation. However, the emphasis is placed in the reverse order. Entrapment is the heart of the industry. It is observable, definable and measurable. The experience of the industry is that petroleum occurs more commonly and in larger quantities in sedimentary basins than in areas of thin and incomplete sedimentary sequences; more commonly and in rather larger quantities in rocks of Mesozoic and Tertiary ages than in older or younger rocks; and more commonly in anticlinal traps than in other types of trap. Experience

also indicates that petroleum is found more cheaply in sedimentarybasins where petroleum has been found before, and in areas where petroleum has been found before.

Oil begins with the bodies of single-celled aquatic organisms. Theancestors of today’s blue-green algae, plankton, and the very delicate,highly symmetrical diatoms, for example, are all thought tohave contributed to the formation of today’s oil. Other determiningfactors are the chemistry of the water in which these organismslived and died and their relative numbers. All oil deposits requiremillions of years to form. They are, therefore, from the point of viewof humanity, irreplaceable.The oil-forming process begins when the bodies of these minute

and ancient creatures are entombed in clays or other very finesediments. The formation of this matrix of organic matter and clayis the first step in the formation of oil. Protected by the clay, theorganic matter is slowly transformed into a material called kerogen.Meanwhile, the twin process of erosion and deposition continueto rework the landscape above thekerogen deposits, changing thedistance to the surface of the clay-kerogen matrix. If erosion predominates

and the kerogen is exposed to the atmosphere, no oil isformed, but if deposition predominates, the kerogen will slowly beburied deeper and deeper. Pressures and temperatures increase with

depth, and if the process is continued long enough, the kerogen willexperience temperatures and pressures conducive to the formationof oil and natural gas. (Oil and gas formation are generally thoughtto occur in the region between 2,500 and 16,000 feet [760–4,900 m]beneath Earth’s surface. Much deeper and only natural gas will beproduced.) These burial and conversion processes require millionsof years, and the details of the entire process—the pressures andtemperatures actually experienced by the kerogen, for example—further affect the quality of the oil produced.

Migration of petroleum into accumulations is thought of largely in terms of permeability pathsMigration is divided into two stages: primary migration from the source rock to the permeable carrier bed; secondary migration from there to the trap, through one or more carrier beds Opinions differ widely on the state of petroleum during migration, whether in solution, in colloidal solution, or as a separate phase in water. Petroleum occurs in solution in formation waters; it occurs as emulsions in the production processes; and it occurs as a separate phase in the trap. Each possibilityhas its merits and its problems.

Migration in aqueous solution has the merit of requiring least work for itstransport through the rocks. The problems relate to the quantitative sufficiency of this process in view of the relatively low solubility of petroleum in water, and the need for some physical or chemical change to be imposed on the solution for the release of petroleum to a separate phase in (or on theway to) the accumulation.

At the other extreme, migration as a separate phase in water has the merit that this is the state found in the accumulation. The problems relate to the mechanical difficulty of transporting petroleum as a separate phase when it is disseminated through the pore spaces with water (and disseminated it must be at some stage between generation and accumulation, because the source

material itself is disseminated). This difficulty is particularly great if primary migration through a fine-grained source rock as a separate phase is postulated. Transport in a colloidal state has the merit that it reduces the difficulties of the alternatives — but it also has a problem relating to the need for an agent to emulsify the petroleum and an agent to deemulsify it. Combinations of these have also been proposed, of which perhaps the most attractive is transport as a separate phase, with residual petroleum being dissolved in the formation water.

Oil reserves are the amount of technically and economically recoverable oil. Reserves may be for a well, for a reservoir, for a field, for a nation, or for the world. Different classifications of reserves are related to their degree of certainty.

Based on data from OPEC at the beginning of 2013 the highest proved oil reserves including non-conventional oil deposits are in Venezuela (20 % of global reserves), Saudi Arabia (18 % of global reserves), Canada (13 % of global reserves), and Iran (9 %).

Because the geology of the subsurface cannot be examined directly, indirect techniques must be used to estimate the size and recoverability of the resource. While new technologies have increased the accuracy of these techniques, significant uncertainties still remain. In general, most early estimates of the reserves of an oil field are conservative and tend to grow with time. This phenomenon is called reserves growth.

All reserve estimates involve uncertainty, depending on the amount of reliable geologic and engineering data available and the interpretation of those data. The relative degree of uncertainty can be expressed by dividing reserves into two principal classifications—«proven» (or «proved») and «unproven» (or «unproved»). Unproven reserves can further be divided into two subcategories—«probable» and «possible»—to indicate the relative degree of uncertainty about their existence. The most commonly accepted definitions of these are based on those approved by the Society of Petroleum Engineers (SPE) and the World Petroleum Council (WPC) in 1997.

Proven reserves are those reserves claimed to have a reasonable certainty (normally at least 90 % confidence) of being recoverable under existing economic and political conditions, with existing technology. Industry specialists refer to this as P90 (i.e., having a 90 % certainty of being produced). Proven reserves are also known in the industry as 1P.

Proven reserves are further subdivided into «proven developed» (PD) and «proven undeveloped» (PUD). PD reserves are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment (operating expense) is required. PUD reserves require additional capital investment (e.g., drilling new wells) to bring the oil to the surface.

Unproven reserves are based on geological and/or engineering data similar to that used in estimates of proven reserves, but technical, contractual, or regulatory uncertainties preclude such reserves being classified as proven. Unproven reserves may be used internally by oil companies and government agencies for future planning purposes but are not routinely compiled. They are sub-classified as probable and possible.

Probable reserves are attributed to known accumulations and claim a 50 % confidence level of recovery. Industry specialists refer to them as «P50" (i.e., having a 50 % certainty of being produced). These reserves are also referred to in the industry as "2P» (proven plus probable).

Possible reserves are attributed to known accumulations that have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10 % certainty of being produced («P10").

Many countries maintain government-controlled oil reserves for both economic and national security reasons. According to the United States Energy Information Administration, approximately 4.1 billion barrels (650,000,000 m3) of oil are held in strategic reserves, of which 1.4 billion is government-controlled (m³=cubic meters). These reserves are generally not counted when computing a nation's oil reserves.

Proved reserves are those quantities of petroleum which, by analysis of geological and engineering data, can be estimated with a high degree of confidence to be commercially recoverable from a given date forward, from known reservoirs and under current economic conditions.

Some statistics on this page are disputed and controversial. Different sources (OPEC, CIA World Factbook, oil companies) give different figures and there are different types of oil, ranging from cheap and easy to recover oils to shale oil or oil sands, which are more expensive and difficult to recover. For example, in the list below, Australia's total amount of oil does not include the shale oil

Because proven reserves includes oil recoverable under current economic conditions, nations may see large increases in proved reserves when known but previously uneconomic deposits become economic to develop. In this way Canada's proven reserves increased suddenly in 2003 when the oil sands of Alberta were seen to be economically viable. Similarly, Venezuela's proven reserves jumped in the late 2000s when the heavy oil of the Orinoco was judged economic.

Reserves amounts are listed in millions of barrels (MMbbl)

 

A 2008 United States Geological Survey estimates that areas north of the Arctic Circle have 90 billion barrels (1.4×1010 m3) of undiscovered, technically recoverable oil and 44 billion barrels (7.0×109 m3) of natural gas liquids in 25 geologically defined areas thought to have potential for petroleum. This represented 13 % of the expected undiscovered oil in the world. Of the estimated totals, more than half of the undiscovered oil resources were estimated to occur in just three geologic provinces—Arctic Alaska, the Amerasia Basin, and the East Greenland Rift Basins. More than 70 % of the mean undiscovered oil resources was estimated to occur in five provinces: Arctic Alaska, Amerasia Basin, East Greenland Rift Basins, East Barents Basins, and West Greenland–East Canada. It was further estimated that approximately 84 % of the oil and gas would occur offshore. The USGS did not consider economic factors such as the effects of permanent sea ice or oceanic water depth in its assessment of undiscovered oil and gas resources. This assessment was lower than a 2000 survey, which had included lands south of the Arctic Circle.

Russian waters in the arctic are expected to contain 100 billion tons of oil and gas

In October 2009, the USGS updated the Orinoco tar sands (Venezuela) value to 513 billion barrels (8.16×1010 m3).

In June 2013 the U. S. Energy Information Administration published a global inventory of estimated recoverable tight oil and tight gas resources in shale formations, «Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States». The inventory is incomplete due to exclusion of tight oil and gas from sources other than shale such as sandstone or carbonates, formations underlying the large oil fields located in the Middle East and the Caspian region, off shore formations, or about which there is little information. Estimated technically recoverable shale oil resources total 335 to 345 billion barrels.

It seems that despite the call for renewable energy there is continuous increase in the quest for oil and gas. With the depletion of reserves onshore the search is increasingly being focused on oceans. The pursuit of oil and gas has driven exploration and production offshore into geographically and geologically complex environments such as ultra-deep waters and the Arctic. These difficult conditions increase the potential risk of accidents that may result in serious marine pollution.

 

Литература:

 

1.    R. E. CHAPMAN Petroleum Geology

2.    Basic Petroleum Geology andLog Analysis

3.    http://en.wikipedia.org/wiki/Oil_reserves