Abstract: A method and apparatus for determining gas corrected porosity using nuclear magnetic resonance (NMR) and density measurements is disclosed. The total porosity (TCMR) or an earth formation is determined using an NMR tool. A log output is generated that indicates the density of the formation. The density porosity of the formation is determined from the log output. The difference between the TCMR-density porosity log accurately indicates the presence of gas. A gas corrected porosity, .phi., and gas saturation, S.sub.g, are determined based upon the density porosity and TCMR.

Abstract: Technique and apparatus are disclosed for identifying characteristics of hydrocarbons, such as distinguishing light and heavy formation oil, in formations surrounding an earth borehole. The technique and apparatus can also be used for determining residual oil saturation, permeability, and bound water volume. An embodiment of the method of the invention includes the following steps: providing a nuclear magnetic resonance logging device that is moveable through the borehole; providing, from the logging device, a polarizing magnetic field and cycles of a magnetic pulse sequence, and receiving magnetic resonance spin echoes from the formations; processing the received spin echoes to produce a T.sub.2 distribution; computing a magnetic resonance porosity from the T.sub.2 distribution; deriving a measure of total porosity of the formations; and comparing the magnetic resonance porosity with the total porosity.

Abstract: A technique is disclosed for determining the gas-filled porosity and oil-filled porosity in a region of investigation of earth formations surrounding a borehole including the following steps: (a) providing a logging device that is moveable through the borehole; (b) generating, from the logging device, a static magnetic field in the region of investigation of the formations, the static field having a magnetic field gradient that is not constant in the region of investigation; (c) determining the gradient distribution of the static magnetic field in the region of investigation, and determining a first function of the gradient distribution and the diffusion coefficient of oil, and a second function of the gradient distribution and the diffusion coefficient of gas; (d) generating a first sequence of magnetic field pulses in the region of investigation of the formations using a first wait time, and detecting a first sequence of nuclear magnetic resonance spin echoes from the formations; (e) generating a second seq

Abstract: An integrated interpretation methodology is described that evaluates carbonate reservoirs. The methodology consists of first classifying the rock facies. A geometrical model specific to the classification is then used to predict the response of the rock to a variety of stimuli. A reconstruction of the geometrical model is made by comparing the measurements with the model predictions. The model is then used to predict the resistivity and the hydraulic transport properties of the rock, thereby enabling computation of both the reserves and their production behavior.

Abstract: In a method for determining the presence of gas in a region of investigation of earth formations surrounding a borehole. A movable logging device is used to generate two sequences of magnetic field pulses having different pulse spacing times. Each of the pulse sequences elicits a corresponding sequence of nuclear magnetic resonance spin echoes. The presence of gas in the formation, is determined from ratios of signals derived from the two spin echoes sequences.

Abstract: A method is disclosed for determining true formation pressure in formations surrounding a fluid-containing borehole having a mudcake on the surface thereof, including the following steps: with the pressure in the borehole at a first measured borehole pressure, measuring, as a first probe pressure, the pressure in the formation adjacent the mudcake; with the pressure in the borehole at a second measured borehole pressure, measuring, as a second probe pressure, the pressure in the formation adjacent the mudcake; and deriving the true formation pressure from the first and second measured borehole pressures and the first and second probe pressures.

Abstract: A method for determining the compensated density of a subsurface geological formation wherein the effect of borehole rugosity is reduced or eliminated utilizes the differences in the vertical response functions of the detectors at different distances into the formation. The method eliminates the contribution of the "shallow" formation region, irrespective of whether its properties are varying slowly or rapidly, without degrading the resulting vertical resolution. Furthermore, the method for determining such a characteristic of a subsurface geological formation does not require any additional or new measurements to be made, thus logging data from older wells can easily be reevaluated.

Abstract: A method for determining a characteristic of a subsurface geological formation enhances the vertical resolution of dual-detector measurements by utilizing a continuous calibration factor which is obtained from an environmentally compensated characteristic derived from at least two resolution-matched sensor signals, and from a resolution-matched sensor signal from the near-detector of the tool. This method also can enhance the statistical precision of the characteristic without degrading the vertical resolution.

Abstract: A method for determining the porosity of a subsurface geological formation traversed by a borehole is provided. The method generally comprises obtaining a means which relates ranges of apparent formation porosity (.phi..sub.a) as determined by a neutron-neutron log, formation fluid salinities, formation total capture cross sections (.SIGMA.), formation matrix and fluid constituents, and true formation porosity (.phi..sub.T) according to a predetermined equation. The predetermined equation relates the apparent porosity to a function of a modified migration length which is obtained in a semi-empirical manner (i.e. physics modifed by data). The modified migration length includes a slowing down length and a diffusion length, but causes at least the diffusion length to be a function of the slowing down length. The means which relates the apparent porosity to salinity, .SIGMA., matrix and fluid constituents, and .phi..sub.T solves a forward problem.

Abstract: A method for determining a characteristic of a subsurface geological formation enhances the vertical resolution of dual-detector measurements by utilizing a continuous calibration factor which is obtained from an environmentally compensated characteristic derived from at least two resolution-matched sensor signals, and from a resolution-matched sensor signal from the near-detector of the tool.

Abstract: A method for determining a characteristic of a subsurface geological formation enhances the vertical resolution of dual-detector measurements by utilizing a continuous calibration factor which is obtained from an environmentally compensated characteristic derived from at least two resolution-matched sensor signals, and from a resolution-matched sensor signal from the near-detector of the tool.

Abstract: Earth formations surrounding a well borehole are analyzed by obtaining a series of capture gamma ray spectra representative of the interaction of thermal neutrons with various chemical elements in the formation surrounding a borehole. The capture gamma ray spectra are used to determine spectral elemental yields quantitatively indicative of the relative abundance of various chemical elements present in the formation. These yields are recorded as a function of depth in said borehole to produce a plurality of traces. The relative sensitivities of the logging tool to a number of the formation components, such as sandstone and limestone, are predetermined either from tool responses in the borehole plus core measurements or from tool responses in a laboratory formation plus the known makeup of the laboratory formation.

Abstract: A method for determining porosity or other formation characteristic of a sub-surface geological formation traversed by a borehole is claimed. A neutron logging tool is passed through the borehole while irradiating the formation with neutrons. The neutron logging tool includes a neutron source and first and second detectors spaced from the source by different distances. Neutrons exiting the formation are detected with the neutron detectors and count rate signals are generated. In response to these signals, an indication of porosity, substantially independent of error due to tool standoff from said borehole wall, is produced. In addition, values of tool standoff are also generated. These standoff values are then filtered by a filter to reduce statistical variations and are used to generate improved indications of porosity.

Abstract: A method and an apparatus for quantitatively determining the macroscopic thermal neutron absorption cross-section of a geological formation traversed by a borehole is disclosed. The method comprises the steps of irradiating the formation with neutrons from a continuous source in a neutron sonde and detecting through the use of dual epithermal and thermal detectors the resultant flux of both thermal and epithermal neutrons. Representative signals are then combined in a manner which best takes advantage of the physics of the interaction of neutrons with matter to produce a representation of the macroscopic thermal neutron absorption cross-section. Thus, a signal representing the ratio of the counts is derived from each like pair of detectors. The logarithms of the signals are then taken and combined in a linear combination.