The oil industry began with onshore fields in the mid-19th century. Even though today’s industry has drilled into ultra-deepwater plays and shale source rocks, onshore fields still form a considerable part of the global oil and gas budget. While reading recent issues of the AAPG Bulletin, I came across some new studies of onshore oil fields in Kazakhstan and China that reminded me of “good old geology” and how it still adds value to science and discovery. Let’s take a look.
Rasoul Sorkhabi
Editor, Core Elements
Carbonates of Kazakhstan
Marko Aliaksandr/Shutterstock.com
A group of Chevron geologists has published two articles about pore types and microfractures in the carbonate reservoirs of the Tengiz and Korolev fields in Kazakhstan.
About the area:
Located along the Caspian shores in western Kazakhstan, the Tengiz Field and smaller Korolev Field are isolated carbonate buildups formed from the Middle Devonian to Lower Pennsylvanian.
The Tengiz was discovered in 1979 and the Korolev in 1986 by the Oil Ministry of the former Soviet Union.
Since 1993, Tengizchevroil, a joint venture between Chevron, ExxonMobil, and KazMunayGas, has operated in both fields.
What’s new:
Many previous studies focused on macroscopic fractures from carbonate reservoirs. New studies from Kazakhstan have detailed on microscopic fractures and pore types.
Researchers used samples from the Middle-Upper Mississippian and Lower Pennsylvanian sections (347 to 315 Ma).
These are the main oil-producing reservoirs in the field.
427 thin-section samples were analyzed by transmitted-light microscopy and laser-scanning confocal microscopy.
Porosity and air permeability measurements were made on core plugs.
634 mercury injection capillary pressure data were collected.
Microfracture classification: The authors propose nine types of microfractures:
Dissolution-enlarged microfractures (most common)
Mosaic microfractures
Sealed and partially sealed microfractures lined with carbonate cement or bitumen
Intercrystalline microfractures in crinoid fragments and coarse blocky calcite cements
En echelon microfractures in low-porosity rocks
Sediment-filled microfractures
Micro-faults and deformation bands
Microfracture “ganglia” (dispersed, denser clusters) in microbial boundstones
Reaction halos (enhanced microporosity) in the matrix near open microfractures
The microfractures have aperture widths ranging from 0.2 to 10 microns.
Microfracture porosity ranges from 0.2 to 1 porosity units (pu).
Pore typing: The researchers identified five pore types from the lowest to highest permeability:
Pore type A: Micropores with a diameter of <10 microns, average porosity 2 pu, and average permeability of 0.05 millidarcy (mD)
Pore type B: Micropores with 30 pu and 0.03 mD
Pore type C: Micropores and macropores with 6 pu and 0.4 mD
Pore type D: Macropores with >10 microns, 5 pu and 0.25 mD
Pore type E: Abundant intercrystalline, micro-rhombic calcite pores with diameters of 2 to 25 microns, average 9.6 pu, and average 1.8 mD
Why it matters: Fractures often provide considerable permeability for oil production from carbonate reservoirs.
Oil fields in the Tarim Basin in China, Xinjian/Shutterstock.com
A group of Chinese scientists has reported on the discovery of a rift basin buried under the Kalpin fold-thrust belt(KFTB)in the northwestern corner of the Tarim Basin.
About the Tarim Basin:
The Tarim Basin in northwest China is a prolific oil and gas basin operated by the China National Petroleum Corporation.
Sandwiched between two mountain ranges—the Tien Shan to the north and the Kunlun to the south—the Tarim Basin has a complex and protracted tectonic history.
KFTB tectonics: Five tectonic events shaped the area of study:
Initial rifting during the Neoproterozoic related to the breakup of the Rodinia supercontinent, followed by sediment deposition during Cryogenian and Ediacaran times
Passive continental margin sedimentation of carbonates in the early Paleozoic
Late Paleozoic collisional tectonics at the southern Tien Shan
Early Mesozoic uplift due to the collision of the Qiantang (North Tibetan) block with the Asian continent
Early Cenozoic collision of the Indian plate with Asia and tectonic reactivation of Asia, including the KFTB
Buried rift basin:
Using seismic images and borehole data, researchers have identified a rift basin buried under the KFTB.
The buried basin-fill is Cryogenian-Ediacaran in age (720–540 Ma).
It appears to be a syn-rift sedimentary wedge.
The initial basin has been deformed by later tectonic events.
It sits beneath a Cambrian shale that has acted as a “thin-skinned” detachment fault associated with several anticlines, indicative of “tectonic inversion.”
Why it matters: The study motivates exploration of deep onshore hydrocarbon resources—some buried under fold-thrust belts—and demonstrates how petroleum exploration reveals so much information about subsurface geology and the tectonic history of the Earth.
Define, and give examples of, the difference between terrain and terrane?
Share your answer by emailing editorial@aapg.org
Answer to last week’s quiz:
Claystone and mudstone are fine-grained sedimentary rocks containing at least two-thirds clay-sized minerals. Some authors consider claystone to be plastic when wet. Shale also has the same clay-rich composition, but it has a fissile or bedding plate texture. Black shales are laminated carbonaceous shale with more than 5 percent organic carbon. Marl is a calcareous (calcite-rich) mudstone. All of these rocks belong to the argillaceous group.
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