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The Elephant in the Room Seal Rock
Categories Blog, exploration

The Elephant in the Room: Seal Rock

Author saifenergy

The Prevailing Suspect: Migration Issues?

My recent work in an area dominated by the belief that source rock and migration pathways are the primary culprits for exploration failures intrigued my curiosity. A quick burial history analysis revealed that even on regional highs, potential and effective source rocks were within the oil window. A 2D regional transect from the recent literature indicated that the source rock is well within the gas window, corroborated by the abundant adjacent seeps. Given that the structure is valid, the reservoirs possess adequate porosity and permeability, and the source rock is in the maturity window with evidence of migration (seepages), what could go wrong in the area?

petroleum system elements

Figure 1 Petroleum System Elements

Timing: Not Always the Villain

Timing is often cited as the main culprit in such scenarios. However, the recent concept of migration lag challenges this notion. Here, timing didn’t seem to be the issue (Figure 1). While examining the petroleum system further, the only remaining element, the “SEAL,” came to the spotlight.

regional high

Figure 2X-Section showing a regional high

The Seal Under Scrutiny

Well logs and mud log painted a concerning picture. The main top seal lacked the typical characteristics of an effective seal (not clay-rich or even shale-rich). Instead, it comprised roughly 300 meters of loose sand, friable sandstone with some shale beds, indicating a low shale gouge ratio and poorly compacted beds with larger pore throats. The overburden pressure was simply not enough to adequately compact the seal rocks. William C. Dawson’s work on the influence of depositional conditions on shale properties and seal character resonated here. It made me question how established regional high (Figure 2) could be explored twice without proper assessment of the top seal’s ability to hold hydrocarbons. Does that mean that shallow reservoir targets are riskier regarding SEAL integrity? Not necessarily.

Why Seal Effectiveness is Often Ignored

Seal effectiveness is often the most challenging element to predict in exploration, hence its frequently neglected. In his LinkedIn blog, Alan Foum mentioned that a significant amount of technical work is done on reservoir prediction using sequence stratigraphy, sedimentology, and reservoir geology. Basin modeling or petroleum systems modeling (and geochemistry) examines the source and migration, reducing the risk on hydrocarbon charge and helping predict the hydrocarbon phase. Seismic interpretation defines the trap, and advanced seismic attributes further aid in reservoir characterization. Yet, seal prediction remains largely ignored. Let’s peg deeper into this issue and find out why it has entrenched in the exploration.

What makes the Seal Conundrum

A study titled “Central North Sea Post Well Analysis” revealed a concerning distribution of exploration failures: Seal (38%), Trap (28%), Reservoir (17%), and Charge (14%). Despite this significantly high number, seal prediction is often ignored due to the industry-wide dogma that seal effectiveness depends solely on facies, thickness, and lithology, ignoring that in all cases, sealing capability depends upon the radius of the pore throat, densities, and interfacial tension. As Zhiyong He explains, the inability to measure seal capacity from seismic data forces explorationists to rely on depositional environment-based ranges and probabilistic migration models. This explains why many geologists, particularly when dealing with thick shale-rich systems, take seals for granted, assuming even a few meters can hold a significant hydrocarbon column. However, thickness alone doesn’t determine seal capacity and column height.

Seal Failure: Friend or Foe? Rethinking a Contentious Concept

Alan Foum references J. Karlo’s video outlining various causes of seal failure (micro-porosity, brittle fractures, hydraulic fracturing, fault displacement, and fault rock composition). While these mechanisms are well-established, recent research has challenged the blanket application of some aspects of the ‘contentious seal failure concept.’ In light of this, let’s scrutinize more closely why seal integrity remains a neglected factor in the oil and gas industry.

Beyond Misconceptions: Rethinking Seal Capacity

Zhiyong He dispels two common misconceptions:

  1. Shales are not impermeable: All shales have some permeability, allowing migration when the capillary displacement pressure is exceeded. Over geological timescales, even very tight shales can accommodate migration.
  2. Leaking seals don’t necessarily mean no hydrocarbons: A seal exceeding its capacity by buoyancy will still trap the maximum column it can hold, just not more. This capacity is typically around 200-400 meters for good marine shale and much less for poor deltaic mudstone.

Conclusion: Seals Deserve Attention

  • Seal rock integrity is a critical yet often overlooked factor in oil and gas exploration.
  • The complexity and difficulty in predicting seal effectiveness is the main reason it is sidelined, despite its significant impact on exploration success.
  • By understanding and examining the data related to seal rocks, explorationists can better evaluate the column a seal can retain.
  • This requires additional data acquisition, which might seem trivial but is highly effective.
  • Would you take SEALS for granted anymore?
References:
  1. Dawson, W. C., Almon, W. R., Dempster, K., & Sutton, S. J. Shale Variability in Deep-Marine Depositional Systems: Implications for Seal Character – Subsurface and Outcrop Examples. (https://www.searchanddiscovery.com/pdfz/documents/2008/08144dawson/ndx_dawson.pdf.html)
  2. Foum, A. (2018). Keeping the hydrocarbons in the trap, effective seals in petroleum exploration. LinkedIn. (https://www.linkedin.com/pulse/keeping-hydrocarbons-trap-effective-seals-petroleum-exploration-foum)
  3. He, Z. (2023). Capillary Seals and Petroleum Migration Observations, Modeling and Risking. (https://www.sodir.no/globalassets/2-force/2023/dokumenter/he-zhiyong-capillary-seals-and-petroleum-migration-force-webinar-nov-2023_final-with-notes.pdf)
  4. Karlo, J. (2018). [Video]. YouTube. (https://www.youtube.com/watch?v=F_YyEpfoj0s)
  5. Moray Firth – Central North Sea Post Well Analysis. (https://www.nstauthority.co.uk/media/1578/cnsmf_post_well_analysis_report.pdf)
Exploring the Offshore Indus Basin Opportuintites
Categories Blog, exploration

Exploring the Offshore Indus Basin: Opportunities and Challenges

Author saifenergy

Exploring The Offshore Indus Basin: Opportunities And Challenges

The Offshore Indus Pak G2-1, the deepest well drilled in the Offshore Indus in terms of water depth, offers interesting insights into the region’s hydrocarbon potential. Despite its depth, the 1D burial history graph (Figure 1) indicates that the well did not achieve the required burial to crack any hydrocarbons. This well, drilled near the Saurashtra Volcanic Arch, encountered a reef, yet remained dry, and the play couldn’t be established.

Reservoir Insights:

Drilling data reveals two sets of proven reservoirs in the Offshore Indus Basin

  • Widely Distributed Miocene Channel Sandstones
  • Locally Distributed Paleocene–Eocene Reef Limestone

Analogies with the adjacent Kutch Basin and the onshore Indus Basin suggest the possible existence of Cretaceous sandstone reservoirs in the Offshore Indus Basin.

Challenges and Historical Context:

The well stopped in the reef limestone after drilling over 200 meters, and unfortunately, it was dry. It was believed that the adjoining synclines would have generated hydrocarbons that would have migrated to the highs (the reefs) but the concept failed. Since the well was stopped early in the reef, remodeling of data in the context of basin is quite difficult. Had the well penetrated the basement, it would have been quite interesting. It was also believed that the reef would be riding all over the basement rocks.  The only proven reservoir is Middle Miocene deltaics that have produced gas in Pak Can-01, but the gas column was too small to justify infrastructure development.

It seems that volcanic activity plays a significant role in the evolution of the offshore part of the Indus Basin and its implications have far-reaching consequences on the hydrocarbon potential.

There have been two major volcanic events in the sea area of Pakistan:

Basalt Eruption of Somnath Ridge (~70 Ma)

Basalt Eruption of Deccan-Reunion (Reunion Mantle Plume, ~65 Ma)

According to Calvès et al. (2010), the basalt eruption of Somnath Ridge contributed to the formation of the volcanic basement in the southeastern Offshore Indus Basin, particularly around Somnath Ridge and Saurashtra High. This area covers approximately 45,000 km².

Geological Insights:

Seismic data interpretation indicates (Figure 2) (that in the southeastern part of the basin adjacent to Somnath Ridge and Saurashtra High, Deccan basalts are distributed in the marine-facies strata of the Upper Cretaceous–Paleocene in a laminated form (Khurram et al., 2019). The northwestern part, far from the Reunion mantle plume, has minimal basalt impact but is close to the strike-slip fault zone of Murray Ridge, making it a potential focus for future oil and gas exploration.

Geothermal Gradients:

Somnath Ridge: Low geothermal gradient of 33℃/km.

Sedimentary Center: High geothermal gradient of 37℃/km – 55℃/km, aiding source rock maturity (Calvès et al., 2010).

The northwestern part, with its developed faults near Murray Ridge, presents an interesting area for future exploration. There are chances that the established Cretaceous plays would be found there (Figure-02).

In most of the wells drilled, the source rocks are in oil window but Pak Can-01 produced gas suggesting that the gas would have been migrated from the deeper part of the basin. Modeling suggests that the Paleocene source rocks (effective in the adjoining onshore) may have become post mature at the end of Oligocene suggesting a charge to the Miocene and younger reservoirs by shallower source rocks.

🔗 Follow us for more insights into the evolving energy landscape and exploration opportunities in the Offshore Indus Basin!

Exploring the Offshore Indus Basin

Figure 2 . After Shahzad et. al.

References:

Calves G, Schwab AM, Huuse M, Peter DC, Asif I.  2010.  Thermal regime of the northwest Indian rifted margin Comparison with Predictions. Marine and Petroleum Geology, 27, 1133–1147.  doi: 10.1016/j.marpetgeo.2010.02.010.

Chatterjee S, Goswami A, Scotese. 2013. The longest voyage: Tectonic, magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia. Gondwana Research, 23, 238–267. doi: 10.1016/j.gr.2012.07.001.

Jian-ming Gonga, b, c, Jing Liaoa, b, *, Jie Lianga, b, Bao-hua Leia, b, Jian-wen Chena, b, Muhammad Khalid, Syed Waseem Haidere, Ming Meng. Exploration prospects of oil and gas in the Northwestern part of the Offshore Indus Basin, Pakistan

Shahzad K, Betzler C, Ahmed N, Qayyum F, Spezzaferri S, Qadir A. 2018. Growth and demise of a Paleogene isolated carbonate platform of the Offshore Indus Basin, Pakistan: Effects of regional and local controlling factors. International Journal of Earth Sciences, 107, 481–504. doi: 10.1007/s00531-017-1504-7.

Rethinking Timing in Petroleum System Analysis: The Role of Migration LagRethinking Timing in Petroleum System Analysis: The Role of Migration Lag
Categories Blog, Drilling, exploration

Rethinking Timing in Petroleum System Analysis: The Role of Migration Lag

Author saifenergy

Rethinking Timing in Petroleum System Analysis: The Role of Migration Lag

Understanding petroleum systems is critical for oil and gas professionals. A common industry assumption holds that the timing of reservoir deposition relative to source rock maturity is crucial. This view suggests reservoirs deposited after oil generation might be barren or gas-prone.

Recent research by Zhiyong He (“Migration Lag – What is it and how it affects Charge Risk and Fluid Properties”) challenges this assumption. He demonstrates that the time between hydrocarbon generation and initial migration (“migration lag”) depends on generation rate and the volume needed to fill the source rock. This lag can represent 10-20% of the hydrocarbon generation window, or even longer for source rocks with reservoir-like properties.

The accompanying diagram (adapted from a Zhiyong He presentation) depicts a typical deep-water Gulf of Mexico burial history, highlighting source rock maturity. The Tithonian source rock entered the oil window 10-20 million years ago. However, the primary reservoirs in this region are middle Miocene, with some as young as Pleistocene. Notably, only in the last 5 million years have Miocene and Pliocene reservoirs begun filling with low-maturity oil. This occurs while the source rock generates gas, flushing out the oil in the initial carrier bed.

Rethinking Timing in Petroleum System Analysis: The Role of Migration LagRethinking Timing in Petroleum System Analysis: The Role of Migration Lag

Furthermore, He cites the Bohai region as another example. Here, the source rock is currently within the gas window, having entered it 10-20 million years ago. Despite this, the main reservoirs in the region hold mostly low-maturity oil.

Rethinking Timing in Petroleum System Analysis: The Role of Migration Lag

Conclusion: These findings suggest that timing may not be as critical a factor in exploration risk assessment as previously believed.

Reference
Migration Lag – What is it and how it affects Charge Risk and Fluid Properties*
Zhiyong He1
Search and Discovery Article #42014 (2017)
https://cva-academy.com/petroleum-systems-analysis-and-modeling.html

World's Largest Metamorphic Rock Oilfield Discovered
Categories Blog, exploration

World’s Largest Metamorphic Rock Oilfield Discovered

Author saifenergy

China Strikes Black Gold in Unexpected Place: World's Largest Metamorphic Rock Oilfield Discovered

In a groundbreaking announcement, the China National Offshore Oil Corporation (CNOOC) revealed the discovery of the world’s largest oilfield in metamorphic rock, situated in the Bohai Sea off the coast of eastern China. The Bohai 26-6 oilfield is located in the southern part of the Bohai Sea (Figure-1), at a relatively shallow depth of about 22 meters. The discovery of an additional 40 MMcmg (1.4 Tcfg) now brings Bohai’s proven reserves to 200 MMcmg (7.1 Tcfg), establishing it as reputedly the largest metamorphic rock oil field in the world (World oil).

Figure 1 Location Map of Bohaj Sea with major features after Hou et al. (2019)

This discovery challenges conventional notions, as the majority of oil and gas reserves are typically found in sedimentary rock formations, unlike the metamorphic rock hosting the Bohai 26-6 reservoir. Metamorphic rock undergoes intense heat and pressure, fundamentally transforming its original form. Traditionally, these rocks were not considered viable candidates for oil and gas exploration. However, the presence of hydrocarbons in this metamorphic reservoir signifies a major breakthrough in independent oil and gas exploration technologies.

Porosity preservation and difference from conventional metamorphic rocks

Due to the scarcity of technical data on the Bohai 26-6 field, attention has shifted towards studying the surrounding area to understand its structural styles, petroleum system, and reservoir characterization. A thorough research paper on the BZ 19-6 field is accessible online, providing detailed information used in this article. Proper citation of the author has been ensured wherever necessary.

The logs in the figure-2 show that there is a separation in the N-D logs against the Archean metamorphic granite rock.  According to Hou et al. (2019), The Archean metamorphic rock is dominated by metamorphic monzonitic granite, metamorphic diorite granite, gneiss and cataclasite intercalated with the later-stage intrusive bodies, such as dioritic porphyrite, ivernite and diabase. 

Figure 2 Reservoir Characterization of drilled section after Hou et al. (2019)

The typical log response in the igneous and metamorphic rocks according to Kansas Geological Survey is as under,

The neutron log

Open pores typically have very low volumes in igneous and metamorphic rocks.

  • low neutron porosity values in acid igneous rocks
  • fairly low neutron porosity values in basic igneous rocks, except for sub aerially weathered basaltic lavas
  • Low values in silica-rich metamorphics but increased values in micaceous rocks and very high values in chlorite schists.

The density log

The bulk density is a valuable diagnostic of igneous and metamorphic rock type.

  • acid igneous rocks have a lower bulk density
  • basic igneous rocks have a much higher bulk density
  • Siliceous metamorphic rocks generally have a lower bulk density than micaceous metamorphic rocks.

In the Archean metamorphic rock, the density is high, and the neutron count, a direct indicator of hydrogen atoms, also increases. However, in the Kongdian Formation, both logs almost overlay. The elevated neutron value suggests additional hydrogen in the logged interval that is provided by either hydrocarbons or sufficient clay content. Additionally, there is notable porosity (5-6%) in the cores, with permeability below 2 milli darcy, as depicted in the figure above. Let’s assess if this aligns with the core details and petrology.

Sidewall cores, thin sections, and SEM analysis reveal well-developed karstification and weathering dissolution zones within the metamorphic rocks. Furthermore, Hou et al. (2019), asserts that there’s evidence suggesting the possibility of cryptoexplosion of supercritical fluid within the Archean buried-hill metamorphic granite. This event could have led to the formation of cryptoexplosive breccia or cryptoexplosive tuff, promoting thermal fluid filtrated alteration. Consequently, high-permeability reservoirs may have formed, containing abundant pores and fractures.

The Bohai 26-6 oilfield is situated primarily within a buried hill (geological feature), its reservoir part comprises of fractured and weathered basement rocks. Additionally, an overlying layer of coarse-grained clastics hints at the potential for additional oil reserves in this stratum. This scenario mirrors similar observations made by Henk Kombrink, GEO ExPro, such as the Lancaster field in the West of Shetlands, UK waters. In such cases, production extends beyond fractured basement rocks to include overlying coarse-grained clastic rocks. It’s intriguing to explore whether this phenomenon repeats itself in China, especially given the thicker upper section.

This find not only holds significant economic potential for China but also represents a crucial step forward in the diversification of global energy sources. As the world continues to navigate the complexities of the energy landscape, the Bohai 26-6 discovery serves as a reminder of the potential for innovation and the ongoing quest for sustainable energy solutions.

Key Points:
  • CNOOC discovered a large oilfield in the Bohai Sea, claimed to be the world’s largest in metamorphic rock.
  • The oil is found in a buried hill, where the reservoir is primarily fractured and weathered basement rock.
  • The presence of overlying coarse-grained clastics raises the possibility of additional oil reserves in this layer.
References
  • Characteristic and controlling factors of deep buried-hill reservoir in the BZ 19-6 structural belt, Bohai sea area:
    Mingcai Hou a b, Haiyang Cao a b, Huiyong Li c, Anqing Chen a b, Ajuan Wei c, Yang Chen a b, Yuechuan Wang c, Xuewei Zhou a b, Tao Ye c
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