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Petrophysics and Well Logs

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Bulletin Article

7969
 

Diagenesis significantly impacts mudstone lithofacies. Processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks. Selected organic-carbon-rich mudstones, from the Kimmeridge Clay and Monterey Formations, have been investigated to determine how varying starting compositions influence diagenesis.

5777
 

The influence of moisture, temperature, coal rank, and differential enthalpy on the methane (CH4) and carbon dioxide (CO2) sorption capacity of coals of different rank has been investigated by using high-pressure sorption isotherms at 303, 318, and 333 K (CH4) and 318, 333, and 348 K (CO2), respectively. The variation of sorption capacity was studied as a function of burial depth of coal seams using the corresponding Langmuir parameters in combination with a geothermal gradient of 0.03 K/m and a normal hydrostatic pressure gradient. Taking the gas content corresponding to 100% gas saturation at maximum burial depth as a reference value, the theoretical CH4 saturation after the uplift of the coal seam was computed as a function of depth. According to these calculations, the change in sorption capacity caused by changing pressure, temperature conditions during uplift will lead consistently to high saturation values. Therefore, the commonly observed undersaturation of coal seams is most likely related to dismigration (losses into adjacent formations and atmosphere). Finally, we attempt to identify sweet spots for CO2-enhanced coalbed methane (CO2-ECBM) production. The CO2-ECBM is expected to become less effective with increasing depth because the CO2-to-CH4 sorption capacity ratio decreases with increasing temperature and pressure. Furthermore, CO2-ECBM efficiency will decrease with increasing maturity because of the highest sorption capacity ratio and affinity difference between CO2 and CH4 for low mature coals.

5774
 

Sandstone pressures follow the hydrostatic gradient in Miocene strata of the Mad Dog field, deep-water Gulf of Mexico, whereas pore pressures in the adjacent mudstones track a trend from well to well that can be approximated by the total vertical stress gradient. The sandstone pressures within these strata are everywhere less than the bounding mudstone pore pressures, and the difference between them is proportional to the total vertical stress. The mudstone pressure is predicted from its porosity with an exponential porosity-versus-vertical effective stress relationship, where porosity is interpreted from wireline velocity. Sonic velocities in mudstones bounding the regional sandstones fall within a narrow range throughout the field from which we interpret their vertical effective stresses can be approximated as constant. We show how to predict sandstone and mudstone pore pressure in any offset well at Mad Dog given knowledge of the local total vertical stress. At Mad Dog, the approach is complicated by the extraordinary lateral changes in total vertical stress that are caused by changing bathymetry and the presence or absence of salt. A similar approach can be used in other subsalt fields. We suggest that pore pressures within mudstones can be systematically different from those of the nearby sandstones, and that this difference can be predicted. Well programs must ensure that the borehole pressure is not too low, which results in borehole closure in the mudstone intervals, and not too high, which can result in lost circulation to the reservoir intervals.

5773
 

This article describes a 250-m (820-ft)-thick upper Eocene deep-water clastic succession. This succession is divided into two reservoir zones: the lower sandstone zone (LSZ) and the upper sandstone zone, separated by a package of pelitic rocks with variable thickness on the order of tens of meters. The application of sequence-stratigraphic methodology allowed the subdivision of this stratigraphic section into third-order systems tracts.

The LSZ is characterized by blocky and fining-upward beds on well logs, and includes interbedded shale layers of as much as 10 m (33 ft) thick. This zone reaches a maximum thickness of 150 m (492 ft) and fills a trough at least 4 km (2 mi) wide, underlain by an erosional surface. The lower part of this zone consists of coarse- to medium-grained sandstones with good vertical pressure communication. We interpret this unit as vertically and laterally amalgamated channel-fill deposits of high-density turbidity flows accumulated during late forced regression. The sandstones in the upper part of this trough are dominantly medium to fine grained and display an overall fining-upward trend. We interpret them as laterally amalgamated channel-fill deposits of lower density turbidity flows, relative to the ones in the lower part of the LSZ, accumulated during lowstand to early transgression.

The pelitic rocks that separate the two sandstone zones display variable thickness, from 35 to more than 100 m (115–>328 ft), indistinct seismic facies, and no internal markers on well logs, and consist of muddy diamictites with contorted shale rip-up clasts. This section is interpreted as cohesive debris flows and/or mass-transported slumps accumulated during late transgression.

The upper sandstone zone displays a weakly defined blocky well-log signature, where the proportion of sand is higher than 80%, and a jagged well-log signature, where the sand proportion is lower than 60%. The high proportions of sand are associated with a channelized geometry that is well delineated on seismic amplitude maps. Several depositional elements are identified within this zone, including leveed channels, crevasse channels, and splays associated with turbidity flows. This package is interpreted as the product of increased terrigenous sediment supply during highstand normal regression.

Bulletin Geologic Note

7961
 

Offshore sequences of volcaniclastic rocks (such as hyaloclastite deposits) are poorly understood in terms of their rock properties and their response to compaction and burial. As petroleum exploration targets offshore volcanic rifted margins worldwide, understanding of volcanic rock properties becomes important both in terms of drilling and how the rocks may behave as seals, reservoirs, or permeability pathways. The Hawaiian Scientific Drilling Project phase II in 2001 obtained a 3 km-(2-mi)-long core of volcanic and volcaniclastic rocks that records the emergence of the largest of the Hawaiian islands. Core recovery of 2945 m (9662 ft) resulted in an unparalleled data set of volcanic and volcaniclastic rocks. Detailed logging, optical petrology, and major element analysis of two sections at depths 1831–1870 and 2530–2597 m (6007–6135 and 8300–8520 ft) are compared to recovered petrophysical logs (gamma ray, resistivity, and P-wave velocity). This study concludes deviation in petrophysical properties does not seem to correlate to changes in grain size or clast sorting, but instead correlates with alteration type (zeolite component) and bulk mineralogy (total olivine phenocryst percentage component). These data sets are important in helping to calibrate well-log responses through hyaloclastite intervals in areas of active petroleum exploration such as the North Atlantic (e.g., Faroe-Shetland Basin, United Kingdom, and Faroe Islands, the Norwegian margin and South Atlantic margins bordering Brazil and Angola).

DL Abstract

3091
 

Earth modeling, from the construction of subsurface structure and stratigraphy, to the accurate understanding of rock physics, through the simulation of seismic and nonseismic responses, is an enabling technology to guide decisions in acquisition, processing, imaging, inversion and reservoir property inference, for both static and time-lapse understanding. So it is crucial to capture those earth elements that most influence the geophysical phenomena we seek to study. This is notoriously difficult, probably because we regularly underestimate how clever the earth can be in producing various geophysical phenomena.

3090
 

Earth modeling, from the construction of subsurface structure and stratigraphy, to the accurate understanding of rock physics, through the simulation of seismic and nonseismic responses, is an enabling technology to guide decisions in acquisition, processing, imaging, inversion and reservoir property inference, for both static and time-lapse understanding. So it is crucial to capture those earth elements that most influence the geophysical phenomena we seek to study. This is notoriously difficult, probably because we regularly underestimate how clever the earth can be in producing various geophysical phenomena.

Explorer Article

8486
 

Robert G. Loucks, a senior research scientist at the Bureau of Economic Geology, University of Texas at Austin, is one of this year’s Robert R. Berg Outstanding Research Award winners. 

8473
 

Shale formations can confound even the savviest geoscientist when it comes to determining the inner workings of the rock. After expert evaluation, even the most attractive prospecting deal can be a tough sell. And there’s almost always a new piece to each of these puzzles that requires some sophisticated high-tech explaining.

Explorer Emphasis

8065
 

Seismic data is an important asset in finding sweet spots in shale plays, as AAPG member Joanne Wang discussed at a recent workshop.

Short Course

Houston Texas United States 02 December, 2014 04 December, 2014 1516
 
Houston, Texas, United States
2-4 December 2014

Here is an introduction to the tools and techniques that geologists and geophysicists use to locate gas and oil, that drillers use to drill the wells and that petroleum engineers use to test and complete the wells and produce the gas and oil. Exercises throughout the course provide practical experience in well log correlation, contouring, interpretation of surface and subsurface, contoured maps, seismic interpretation, well log interpretation, and decline curve analysis.

Austin Texas United States 29 April, 2014 01 May, 2014 1518
 
Austin, Texas, United States
29 April - 1 May 2014

The overall goal of this course is to provide tools for efficient and effective re-exploration and development. It uses a two-part approach. First it uses petrophysical analysis to understand all that can be derived from examination of standard open-hole logs. This is followed by integrated approaches to discover key factors controlling oil and gas distribution in carbonate reservoirs in the greater Midcontinent USA. Methodologies and workflows reviewed include geosteering and evaluation of horizontal wells and optimizing carbon storage utilization and management.

Houston Texas United States 19 August, 2014 21 August, 2014 1509
 
Houston, Texas, United States
19-21 August 2014

Here is an introduction to the tools and techniques that geologists and geophysicists use to locate gas and oil, that drillers use to drill the wells and that petroleum engineers use to test and complete the wells and produce the gas and oil. Exercises throughout the course provide practical experience in well log correlation, contouring, interpretation of surface and subsurface, contoured maps, seismic interpretation, well log interpretation, and decline curve analysis.

Denver Colorado United States 03 June, 2014 05 June, 2014 1505
 
Denver, Colorado, United States
3-5 June 2014

Here is an introduction to the tools and techniques that geologists and geophysicists use to locate gas and oil, that drillers use to drill the wells and that petroleum engineers use to test and complete the wells and produce the gas and oil. Exercises throughout the course provide practical experience in well log correlation, contouring, interpretation of surface and subsurface, contoured maps, seismic interpretation, well log interpretation, and decline curve analysis.

Tulsa Oklahoma United States 07 October, 2014 09 October, 2014 1511
 
Tulsa, Oklahoma, United States
7-9 October 2014

Here is an introduction to the tools and techniques that geologists and geophysicists use to locate gas and oil, that drillers use to drill the wells and that petroleum engineers use to test and complete the wells and produce the gas and oil. Exercises throughout the course provide practical experience in well log correlation, contouring, interpretation of surface and subsurface, contoured maps, seismic interpretation, well log interpretation, and decline curve analysis.

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