Crude Oil Accumulations Research Articles

Near-Surface Evidence of Migration of Natural Gas from Deep Reservoirs and Source Rocks

Gaseous hydrocarbons migrate to the surface in small quantities from deep source rocks and natural gas or crude oil accumulations. This process leads to anomalous concentrations of hydrocarbons in near-surface sediments. The hydrocarbon anomalies (amount and composition) are useful in exploration, because they may point to oil and gas reservoirs. The origin of hydrocarbon anomalies in shallow sediments is not a simple one. Methane in near-surface sediments may be caused by a combination of bacterial production of methane, leakage from reservoirs, and leakage of methane from thermal maturation of organic matter in source rocks. Organic geochemistry, especially carbon isotope techniques, have been used to recognize secondary fractionation processes caused by degassing or bacterial oxidation. Examples of geochemical surveys in onshore and offshore areas indicate secondary fractionations so small that gases from near-surface sediments and from reservoirs can be correlated by the geochemical data. The application of stable carbon isotopes is a significant step toward improving geochemical hydrocarbon surface exploration methods.

Solubility and mobility of petroleum under hydrodynamic conditions, Surat Basin, Queensland

Abstract Factors relating to the solubility of hydrocarbons in formation water, with particular reference to the Precipice Sandstone of the Surat Basin, are discussed. It is suggested that a possible mechanism for the migration and accumulation of crude oil and natural gas in the Surat Basin has been the solution and transportation of hydrocarbons in formation water under hydrodynamic conditions. The Moonie structure is presented as a model.

Producing Viscous Fluids

Abstract The discovery in March, 1946, of the Boscan low gravity-high viscosity crude oil accumulation presented the Richmond Exploration Co. with a multitude of problems almost unique in the industry. Solution of these problems through experience and cut-and-try methods has resulted in the successful development of a large source of rod-building materials. Introduction Among the oil companies operating in Venezuela, to which heavy crude means some 14° to 15° API gravity oil, the name Boscan has practically become a synonym of "viscous fluid." Indeed, Boscan crude has an average 10° API gravity fluctuating from 9° to 12° and an average viscosity of 26,000 SSU at 122°F fluctuating from 17,000 to 50,000 SSU. This asphalt-looking oil has to be pumped out of the ground, shipped 25 miles away and loaded on tankers. When the Boscan field was discovered, the technical obstacles brought up by the physical characteristics of its crude made questionable the commercial exploitation of a petroleum so difficult to produce. However, three years later, Richmond was shipping 14,000 B/D of Boscan crude. Even if the figure does not seem impressive, it means the practical answers to a large sequence of major production problems which had to be solved within the rates of the available equipment through constantly improving methods and designs. The drilling of a Boscan well is not a difficult task; it is when the rig is removed that obstacles start arising. Geology The Boscan field is located 26 miles southwest of Maracaibo in the State of Zulia in the western part of the Maracaibo basin. The stratigraphic sequence encountered in and above the production section in the Boscan area includes Eocene, Oligocene, Miocene and Pliocene sediments. The Upper Eocene consists of the Las Flores formation with interbedded sands and dark gray carbonaceous shales. In the upper part, two zones of beach and bar sands (designated as Upper Boscan and Lower Boscan) produce heavy oil. The Lower Oligocene overlies the Eocene and contains poor to well developed beach sands interbedded with light gray sandy claystone. Effectively permeable sands in this formation (designated Basal Icotea) locally are oil productive. The middle of Oligocene, principally claystone, contains thin sands which are oil productive in the eastern part of the field. This is Middle Icotea.