BLACK SHALES AND PHOSPHORITES, RESULTS OF DIFFERENTIAL DIAGENETIC EVOLUTION OF ORGANIC MATTER IN THE OULAD ABDOUN-TIMAHDIT SEDIMENTARY BASIN (MOROCCO)

Carbonate units of two facie of the platformal upper reef complex of the Late Silurian West Point Formation, Gaspé Peninsula, Quebec, were studied to decipher their diagenetic evolution. The two facies were a reef-margin facies and a back-reef to lagoonal facies. Under the light microscope, only three broad cement phases were recognized. In contrast, cathodoluminescence observation revealed seven distinct generations of cements and a plethora of additional diagenetic features such as fracturing, internal brecciation, sulfatization, and stylolitization. The first four generations of cement were early marine and confined to the reef-margin facies. The subsequent three cement generations evolved in shallow to deeper burial environments and affected the reef-margin facies and the back-reef to lagoonal facies. C and O stable-isotope data support these deductions. Fracturing, internal brecciation, stylolitization, sulfatization, and the generation of the latest cements occurred during compaction and postcompaction stages. Lithification of the facies was rapid, with the pores completely occluded before a maximum burial depth of about 1 km was attained (based on conodont colour-alteration indices, organic-matter maturation data, and overall post-Silurian paleogeography).

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Molecular and petrographical evidence for lacustrine environmental and biotic change in the palaeo-Sichuan mega-lake (China) during the Toarcian Oceanic Anoxic Event

Geological Society London Special Publications ◽

10.1144/sp514-2021-2 ◽

2021 ◽

pp. SP514-2021-2

Author(s):

Weimu Xu ◽

Johan W. H. Weijers ◽

Micha Ruhl ◽

Erdem F. Idiz ◽

Hugh C. Jenkyns ◽

...

Keyword(s):

Organic Matter ◽

Sichuan Basin ◽

Large Scale ◽

Algal Growth ◽

Black Shales ◽

Content Type ◽

Oceanic Anoxic Event ◽

Anoxic Event ◽

Supplementary Material ◽

The Sichuan Basin

AbstractThe organic-rich upper Lower Jurassic Da'anzhai Member (Ziliujing Formation) of the Sichuan Basin, China is the first stratigraphically well-constrained lacustrine succession associated with the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma). The formation and/or expansion of the Sichuan mega-lake, likely one of the most extensive fresh-water systems to have existed on the planet, is marked by large-scale lacustrine organic productivity and carbon burial during the T-OAE, possibly due to intensified hydrological cycling and nutrient supply. New molecular biomarker and organic petrographical analyses, combined with bulk organic and inorganic geochemical and palynological data, are presented here, providing insight into aquatic productivity, land-plant biodiversity, and terrestrial ecosystem evolution in continental interiors during the T-OAE. We show that lacustrine algal growth during the T-OAE accounted for a significant organic-matter flux to the lakebed in the palaeo-Sichuan mega-lake. Lacustrine water-column stratification during the T-OAE facilitated the formation of dysoxic-anoxic conditions at the lake bottom, favouring organic-matter preservation and carbon sequestration into organic-rich black shales in the Sichuan Basin. We attribute the palaeo-Sichuan mega-lake expansion to enhanced hydrological cycling in a more vigorous monsoonal climate in the hinterland during the T-OAE greenhouse.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5433544

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Lithofacies, Depositional Environment and Diagenetic Evolution of the Paleocene Patala Formation, Potwar Basin, Pakistan: Implication for Shale Gas Potential

10.5194/egusphere-egu21-12555 ◽

2021 ◽

Author(s):

Nasar Khan ◽

Rudy Swennen ◽

Gert Jan Weltje ◽

Irfan Ullah Jan

Keyword(s):

Organic Matter ◽

Shale Gas ◽

Gas Reservoirs ◽

Shallow Marine ◽

Anoxic Conditions ◽

Fine Grained ◽

Potwar Basin ◽

Diagenetic Evolution ◽

Reservoir Assessment ◽

Gas Potential

Abstract: Reservoir assessment of unconventional reservoirs poses numerous exploration challenges. These challenges relate to their fine-grained and heterogeneous nature, which are ultimately controlled by depositional and diagenetic processes. To illustrate such constraints on shale gas reservoirs, this study focuses on lithofacies analysis, paleo-depositional and diagenetic evolution of the Paleocene Patala Formation at Potwar Basin of Pakistan. Integrated sedimentologic, petrographic, X-ray diffraction and TOC (total organic carbon) analyses showed that the formation contained mostly fine-grained carbonaceous, siliceous, calcareous and argilaceous siliciclastic-lithofacies, whereas carbonate microfacies included mudstone, wackestone and packstone. The silicious and carbonaceous lithofacies are considered a potential shale-gas system. The clastic lithofacies are dominated by detrital and calcareous assemblage including quartz, feldspar, calcite, organic matter and clay minerals with auxiliary pyrites and siderites. Fluctuations in depositional and diagenetic conditions caused&#160; lateral and vertical variability in lithofacies. Superimposed on the depositional heterogeneity are spatially variable diagenetic modifications such as dissolution, compaction, cementation and stylolitization. The &#948;13C and &#948;15N stable isotopes elucidated that the formation has been deposited under anoxic conditions, which relatively enhanced the preservation of mixed marine and terrigenous organic matter. Overall, the Patala Formation exemplifies deposition in a shallow marine (shelfal) environment with episodic anoxic conditions.Keywords: Lithofacies, Organic Matter, Paleocene, Potwar Basin, Shale Gas, Shallow Marine.

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