Hello everyone! I just wrapped up final exams here at The University of Utah. Another year in the books! Meanwhile, Core Elements is jumping into May with two strong studies published in AAPG journals—both looking at geological elements of the Sichuan Basin.

Happy reading! And don’t hesitate to shoot me a note with any ideas for topics you would like to see in the next Core Elements. I can be reached at editorial@aapg.org.

Rasoul Sorkhabi

Editor, Core Elements

Natural Gas in the World’s Deepest Volcanic Reservoir

Courtesy of AAPG Bulletin

Researchers from Chengdu recently published an article around the occurrence of natural gas in two volcanic reservoirs in the Sichuan basin. The paper sheds light on how igneous reservoirs work.

Reservoir background:

• The vast majority of petroleum reservoirs are found in sandstone or limestone, but nearly a quarter are found in igneous rocks.

• How petroleum gets into non-sedimentary rocks is an interesting story, but before we stretch your imagination around deep mantle-derived petroleum, I should note: igneous reservoirs are usually hosted in sedimentary basins and are fed by sedimentary source rocks.

A tale of two volcanic reservoir rocks: Here are some key points from the new case study.

• Age: The two volcanic reservoirs—named Jianyang and Zhougongshan—are both Permian age, but their petroleum systems differ.

• Depth: These Permian natural gas reservoirs are the deepest (>5000 meters) volcanic reservoir ever reported.
• Tectonics: Both volcanic reservoirs were fractured by tectonic activities to create sufficient porosity to store hydrocarbons.

• Sources: Natural gas in Jianyang was fed from the Cambrian source rocks inside the basin, while Zhougongshan was charged by the Permian source rock.

• Migration: Natural gas in Jianyang travelled from deeper rocks, and its migration required efficient pathways. Oil in Zhougongshan did not require long migration.

• Porosity and permeability: One might think that Zhougongshan would be a better prospect, but it is actually a poor reservoir with low porosity and permeability. Jianyang has high porosity and permeability and is highly pressurized—all green flags for gas production.

• Petroleum traps: In Jianyang, stratigraphic traps spatially controlled the distribution of fractured basaltic rocks, while in Zhougongshan, structural traps were breached and leaked by multistage tectonic activities.

Two big takeaways:

• This new study offers some critical guidelines for exploration of volcanic reservoirs.

• The same reservoir formation may behave differently because of differences in geological development.

Go deeper: Read the study by Di Xia and colleagues here.

A message from AAPG DPA and SPE OGRC

The AAPG DPA and the SPE OGRC are proud to offer a 1-day short course
‘Resources Evaluation Using PRMS’
Class approved by SPE Oil and Gas Reserves Committee

Join AAPG and SPE on 23 May for a short course that will provide a detailed description of the 2018 PRMS update, including resources classes, categories, and status; resources assessment methods; key changes in the 2018 update (compared to the earlier 2007 PRMS); and navigating in the PRMS document itself.

Learn more and register here.

Shale we Discuss Tensile Strength?

Kimtaro/Shutterstock.com

We will still stay close to the Sichuan basin but branch into shale next. Delu Li and colleagues from Xian University of Science and Technology recently reported on the main factors controlling tensile strength in shale reservoirs.

Let’s break it down: Tensile strength measures how much stretching stress (or load) a rock can support before it fractures.

What they did:

• The researchers collected 12 shale samples from the Cambrian-age Niutitang formation outcropped to the north of the Sichuan Basin.

• They measured tensile strength of the sample using tri-axial tester and also collected data on mineral composition, pore distribution, and total organic carbon.

What they found:

• Tensile strength of shale samples ranging from 10–20 MPa is high and comparable to sandstone.

• Quartz had the largest proportion of shale minerals at 53–59 percent.

• Total organic carbon ranged from 1.7–4.1 percent, but it showed little effect on the tensile strength.

• Pore diameters averaged 12.6–16.00 nanometers.

• Porosity distribution analyzed by NMR showed the largest proportion for meso-porosity (76–88 percent) residing in quartz, followed by macro-porosity (13–21 percent), and finally micro-porosity (0.1–5 percent) in clay minerals.

• Lower tensile strength coincided with higher proportion of mesopores. 

What makes this study stand out:

• Although tensile strength is a key mechanical property for reservoir engineering and shale production that requires hydraulic fracturing, data on fracture strength of rocks require rigorous laboratory experiments and are not abundant.
• Additionally, the study by Li and colleagues has combined tensile strength with other geological parameters for relational analysis.

Read more: Find the full study here.

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