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Bulletin E-P Note

Athabasca oil sands: Megatrap restoration and charge timing

The petroleum trap for the Athabasca oil sands has remained elusive because it was destroyed by flexural loading of the Western Canada Sedimentary Basin during the Late Cretaceous and Paleocene. The original trap extent is preserved because the oil was biodegraded to immobile bitumen as the trap was being charged during the Late Cretaceous. Using well and outcrop data, it is possible to reconstruct the Cretaceous overburden horizons beyond the limit of present-day erosion. Sequential restoration of the reconstructed horizons reveals a megatrap at the top of the Wabiskaw-McMurray reservoir in the Athabasca area at 84 Ma (late Santonian). The megatrap is a four-way anticline with dimensions 285 x 125 km (177 x 78 mi) and maximum amplitude of 60 m (197 ft). The southeastern margin of the anticline shows good conformance to the bitumen edge for 140 km (87 mi). To the northeast of the anticline, bitumen is present in a shallower trap domain in what is interpreted to be an onlap trap onto the Canadian Shield; leakage along the onlap edge is indicated by tarry bitumen outliers preserved in basement rocks farther to the northeast. Peripheral trap domains that lie below the paleospillpoint, in northern, southern, and southwestern Athabasca, and Wabasca, are interpreted to represent a late charge of oil that was trapped by bitumen already emplaced in the anticline and the northeastern onlap trap. This is consistent with kimberlite intrusions containing live bitumen, which indicate that the northern trap domain was charged not before 78 Ma. The trap restoration has been tested using bitumen-water contact well picks. The restored picks fall into groups that are consistent both with the trap domains determined from the top reservoir restoration and the conceptual charge model in which the four-way anticline was filled first, followed by the northeastern onlap trap, and then the peripheral trap domains.

Added on 28 February, 2014

Explorer Historical Highlights By Michael Morton

A Gusher at Baba Gurgur

“One man was at the head waters of the River Amazon among the headshrinkers when he was recalled to come to Iraq; another came from Argentina, another from Mexico, still another had been in Romania, one in Indo-China, several in Venezuela and the East Indies.”

Added on 01 February, 2014

Explorer Article By Bob Krantz,Brett Freeman,Carol Ormand

Geologists Get a Closer 'Look' at Complexity

Geologists, cognitive scientists, software developers and other industry stakeholders put their heads together at a recent Hedberg conference to share insights on the 3-D interpretation and spatial thinking skills that are vital to geoscience.

Added on 01 February, 2014

Bulletin Article

Evaluation of hydrocarbon generation and migration in the Molasse fold and thrust belt (Central Eastern Alps, Austria) using structural and thermal basin models

The Molasse Basin represents the northern foreland basin of the Alps. After decades of exploration, it is considered to be mature in terms of hydrocarbon exploration. However, geological evolution and hydrocarbon potential of its imbricated southernmost part (Molasse fold and thrust belt) are still poorly understood. In this study, structural and petroleum systems models are integrated to explore the hydrocarbon potential of the Perwang imbricates in the western part of the Austrian Molasse Basin. The structural model shows that total tectonic shortening in the modeled north–south section is at least 32.3 km (20.1 mi) and provides a realistic input for the petroleum systems model. Formation temperatures show present-day heat flows decreasing toward the south from 60 to 41 mW/m2. Maturity data indicate very low paleoheat flows decreasing southward from 43 to 28 mW/m2. The higher present-day heat flow probably indicates an increase in heat flow during the Pliocene and Pleistocene. Apart from oil generated below the imbricated zone and captured in autochthonous Molasse rocks in the foreland area, oil stains in the Perwang imbricates and oil-source rock correlations argue for a second migration system based on hydrocarbon generation inside the imbricates. This assumption is supported by the models presented in this study. However, the model-derived low transformation ratios (20%) indicate a charge risk. In addition, the success for future exploration strongly depends on the existence of migration conduits along the thrust planes during charge and on potential traps retaining their integrity during recent basin uplift.

Added on 31 January, 2014

Bulletin Article

Fault displacement-distance relationships as indicators of contractional fault-related folding style

We present a method of using fault displacement-distance profiles to distinguish fault-bend, shear fault-bend, and fault-propagation folds, and use these insights to guide balanced and retrodeformable interpretations of these structures. We first describe the displacement profiles associated with different end-member fault-related folding models, then provide examples of structures that are consistent with these model-based predictions. Natural examples are imaged in high-resolution two- and three dimensional seismic reflection data sets from the Niger Delta, Sichuan Basin, Sierras Pampeanas, and Cascadia to record variations in displacement with distance updip along faults (termed displacement-distance profiles). Fault-bend folds exhibit constant displacement along fault segments and changes in displacement associated with bends in faults, shear fault-bend folds demonstrate an increase in displacement through the shearing interval, and fault-propagation folds exhibit decreasing displacement toward the fault tip. More complex structures are then investigated using this method, demonstrating that displacement-distance profiles can be used to provide insight into structures that involve multiple fault-related folding processes or have changed kinematic behavior over time. These interpretations are supported by comparison with the kinematics inferred from the geometry of growth strata overlying these structures. Collectively, these analyses illustrate that the displacement-distance approach can provide valuable insights into the styles of fault-related folding.

Added on 31 January, 2014

Bulletin Article

A surprising asymmetric paleothermal anomaly around El Gordo diapir, La Popa Basin, Mexico

Thirty-seven mudstone samples were collected from the uppermost Lower Mudstone Member of the Potrerillos Formation in El Gordo minibasin within La Popa Basin, Mexico. The unit is exposed in a circular pattern at the earth's surface and is intersected by El Gordo diapir in the northeast part of the minibasin. Vitrinite reflectance (Ro) results show that samples along the eastern side of the minibasin (i.e., south of the diapir) are mostly thermally immature to low maturity (Ro ranges from 0.53% to 0.64%). Vitrinite values along the southern, western, and northwestern part of the minibasin range between 0.67% and 0.85%. Values of Ro immediately northwest of the diapir are the highest, reaching a maximum of 1.44%. The results are consistent with two different possibilities: (1) that the diapir plunges to the northwest, or (2) that a focused high-temperature heat flow existed along just the northwest margin of the diapir. If the plunging diapir interpretation is correct, then the thermally immature area south of the diapir was in a subsalt position, and the high-maturity area northwest of the diapir was in a suprasalt position prior to Tertiary uplift and erosion. If a presumed salt source at depth to the northwest of El Gordo also fed El Papalote diapir, which is located just to the north of El Gordo diapir, then the tabular halokinetic sequences that are found only along the east side of El Papalote may be subsalt features. However, if the diapir is subvertical and the high-maturity values northwest of the diapir are caused by prolonged, high-temperature fluid flow along just the northwestern margin of the diapir, then both of these scenarios are in disagreement with previously published numerical models. This disagreement arises because the models predict that thermal anomalies will extend outward from a diapir a distance roughly 1.5 times the radius of the diapir, but the results reported here show that the anomalous values on one side of the diapir are about two times the radius, whereas they are as much as five times the radius on the other side of the diapir. The results indicate that strata adjacent to salt margins may experience significantly different heat histories adjacent to different margins of diapirs that result in strikingly different diagenetic histories, even at the same depth.

Added on 31 January, 2014

Bulletin Article

Salt structures and hydrocarbon accumulations in the Tarim Basin, northwest China

The Tarim Basin is one of the most important hydrocabon-bearing evaporite basins in China. Four salt-bearing sequences, the Middle and Lower Cambrian, the Mississippian, the Paleogene, and the Neogene, have various thickness and areal distribution. They are important detachment layers and intensely affect the structural deformation in the basin. The Kuqa depression is a subordinate structural unit with abundant salt structures in the Tarim Basin. Salt overthrusts, salt pillows, salt anticlines, salt diapirs, and salt-withdrawal basins are predominant in the depression. Contraction that resulted from orogeny played a key function on the formation of salt structures. Growth strata reveal that intense salt structural deformation in the Kuqa depression occurred during the Himalayan movement from Oligocene to Holocene, with early structural deformation in the north and late deformation in the south. Growth sequences also record at least two phases of salt tectonism. In the Yingmaili, Tahe, and Tazhong areas, low-amplitude salt pillows are the most common salt structures, and these structures are commonly accompanied by thrust faults. The faulting and uplifting of basement blocks controlled the location of salt structures. The differences in the geometries of salt structures in different regions show that the thickness of the salt sequences has an important influence on the development of salt-cored detachment folds and related thrust faults in the Tarim Basin. Salt sequences and salt structures in the Tarim Basin are closely linked to hydrocarbon accumulations. Oil and gas fields have been discovered in the subsalt, intrasalt, and suprasalt strata. Salt deformation has created numerous potential traps, and salt sequences have provided a good seal for the preservation of hydrocarbon accumulations. Large- and small-scale faults related with salt structures have also given favorable migration pathways for oil and gas. When interpreting seismic profiles, special attention needs to be paid to the clastic and carbonate interbeds within the salt sequences because they may lead to incorrect structural interpretation. In the Tarim Basin, the subsalt anticlinal traps are good targets for hydrocarbon exploration.

Added on 31 December, 2013

Explorer Article By Louise Durham

Tool Aids Interpretation

Breaking from the manual picking methods, 'Geological Expression' interprets complex structural information from 3-D seismic. The result is faster interpretation yielding data that can be quickly manipulated to extract real geological features.

Added on 01 December, 2013

Bulletin Article

Tectonic versus diagenetic origin of fractures in a naturally fractured carbonate reservoir analog (Nerthe anticline, southeastern France)

Field analogs allow a better characterization of fracture networks to constrain naturally fractured reservoir models. In analogs, the origin, nature, geometry, and other attributes of fracture networks can be determined and can be related to the reservoir through the geodynamic history. In this article, we aim to determine the sedimentary and diagenetic controls on fracture patterns and the genetic correlation of fracture and diagenesis with tectonic and burial history. We targeted two outcrops of Barremian carbonates located on both limbs of the Nerthe anticline (southeastern France). We analyzed fracture patterns and rock facies as well as the tectonic, diagenetic, and burial history of both sites. Fracture patterns are determined from geometric, kinematic, and diagenetic criteria based on field and lab measurements. Fracture sequences are defined based on crosscutting and abutting relationships and compared with geodynamic history and subsidence curves. This analysis shows that fractures are organized in two close-to-perpendicular joint sets (i.e., mode I). Fracture average spacing is 50 cm (20 in.). Fracture size neither depends on fracture orientation nor is controlled by bed thickness. Neither mechanical stratigraphy nor fracture stratigraphy is observed at outcrop scale. Comparing fracture sequences and subsidence curves shows that fractures existed prior to folding and formed during early burial. Consequently, the Nerthe fold induced by the Pyrenean compression did not result in any new fracture initiation on the limbs of this fold. We assume that the studied Urgonian carbonates underwent early diagenesis, which conferred early brittle properties to the host rock.

Added on 30 November, 2013

Explorer Article

Four DLs Have Speaking Dates In November

The Distinguished Lecture program, funded in part by the AAPG Foundation, is the Association’s flagship initiative for spreading the latest in science, technology and professional information.

Added on 01 November, 2013

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Added on 01 April, 2024

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Added on 26 March, 2024

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Added on 01 March, 2022

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Added on 01 April, 2024

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Introducing the New EXPLORER, New Digital Newsletters

Perhaps you did a double take pulling the April issue of EXPLORER from the mailbox. What is this? If you joined AAPG in the last 40 years, you’ve only known EXPLORER in its long-standing tabloid format. It worked well for many years as our advertisers – particularly seismic companies – loved the large format and the ability to display their data on a sweeping canvas. For readers, it was a little more awkward.

Added on 01 April, 2024

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