The Qamchuqa Formation is well-exposed in the High Folded Zone (HFZ) of northeast Iraq. The formation is highly dolomitized and contains thick reservoir units in the subsurface region south of the ...HFZ. Deposition of the formation took place at the shelf margin of Early Cretaceous Neo-Tethys Ocean. It merges south- and southeastward to the shallow marine shelf of Arabia and to deep basin accumulations farther north. In the study area, the formation is divided into three members: lower member (250m thick) of interbedded bioclastic calcimudstone/wackestone (MW), bioclastic packstone (BP), bioclastic floatstone (BF) and medium-to coarse-crystalline dolorudstone/dolograinstone (CD). These strata accumulated in low to medium energy, subtidal, fore-shoal setting. The middle member consists of thick succession (325m) of massively bedded CD lithofacies interpreted as high energy shoal facies which represents Barremian-Aptian main shelf-margin succession. The upper member (155m thick) consists of three lithofacies of MW, CD and fine crystalline dolomudstone (FD), in ascending order. It is interpreted as lagoon (MW) through subtidal carbonate sand shoal (CD) to supratidal (penecontemporaneous) dolomudstone (FD). The lower two members represent a long-term Barremian to Aptian shallowing-upward megacycle capped by late Aptian unconformity incurred by relative sea level drop. This temporal gap is coeval with an erosional vacuity recognized throughout the Arabian Peninsula. Sea level rise in early Albian heralded deposition of the upper member which also represents a shallowing-upward cycle surmounted by regionally extensive subaerial unconformity recognized in the Arabian Peninsula. Correlation and stratigraphic position of the formation under regional context has been discussed.
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•Qamchuqa Fm. is an Early Cretaceous stratigraphic unit which accumulated at the southern margin of the Neo-Tethys Ocean.•It is divided into three members of which the lower two represent Barremian to Aptian fore-shoal and shoal facies.•The upper member lies unconformably over the middle member and represents Albian depositional system.•Stratigraphic locations of the three members with the context of regional correlation have been documented.•Cyclostratigraphy, relative sea level changes & regional unconformities at the Neo-Tethys shelf margin have been discussed.
The Albian Kharfot Formation is preserved in the eastern margin of the Jeza-Qamar Basin which straddles across the Oman–Yemen border. This study addresses the sedimentological attributes of the ...formation and deduces its depositional setting, cyclicity and relative sea level changes in local (within the basin) and regional (Arabian) contexts. The interaction among siliciclastic influxes, in-situ carbonate production and tectono-climatic controls on the stacking nature of the various lithofacies that build-up the formation is discussed. In the study area, the formation lies unconformably over Barremian–Aptian Qishn Formation and conformably under late Albian–?Turonian Dhalqut Formation. The Kharfot Formation thickens from ~140m in the eastern side of the study area to ~300m at the Oman–Yemen border. It consists of eight lithofacies: Orbitolina-rich marls, peloidal bioclastic packstone, bioclastic mudstone to wackestone, argillaceous, bioclastic floatstone to rudstone, bioclastic rudstone, sandy, peloidal, bioclastic mudstone to packstone, peloidal, bioclastic grainstone and dolostone. The vertical arrangement of these lithofacies defines recurring meter- to decameter-scale, shallowing-upward units deposited on a westward-deepening inner- to outer-ramp setting. Tectonic rejuvenation of the siliciclastic source area was accompanied by warm, humid climatic conditions as suggested by high kaolinitic marls of the Kharfot Fm. and coeval quartz-rich sandstone units (Harshiyat Fm.). The shallowing-upward rhythmic sedimentation of the formation has close resemblance with cycles of the Nahr Umr Formation in northern Oman and partially comparable with the global sea level changes. The Kharfot basin was an intrashelf depression that was part of the much larger Arabian epeiric platform. The latter is defined by a rimmed margin in northern Oman where Al-Hassanat Formation represents platform margin deposits and Nahr Umr Formation representing back-rim intrashelf depression which received fine-grained siliciclastic influx from the land (westward). In southern Oman where Kharfot Formation accumulated, the platform was unrimmed ramp type basin with high fine clastic influx along with elevated carbonate production.
•Lithofacies and depositional environment of the Albian Kharfot Formation, Oman.•Concurrent siliciclastic and carbonate deposition within regional (Arabian) context.•Cyclicity, tectono-eustatic fluctuations and their effect on basin-fill architecture.
The quality of source rocks plays an important role in the distribution of tight and conventional oil and gas resources. Despite voluminous studies on source rock hydrocarbon generation, expulsion ...and overpressure, a quality grading system based on hydrocarbon expulsion capacity is yet to be explored. Such a grading system is expected to be instrumental for tight oil and gas exploration and sweet spot prediction. This study tackles the problem by examining Late Cretaceous, lacustrine source rocks of the Qingshankou 1 Member in the southern Songliao Basin, China. By evaluating generated and residual hydrocarbon amounts of the source rock, the extent of hydrocarbon expulsion is modelled through a mass balance method. The overpressure is estimated using Petromod software. Through correlation between the hydrocarbon expulsion and source rock evaluation parameters total organic carbon (TOC), kerogen type, vitrinite reflectance (Ro) and overpressure, three classes of high-quality, effective and ineffective source rocks are established. High-quality class contains TOC >2%, type-I kerogen, Ro >1.0%, overpressure >7Mpa, sharp increase of hydrocarbon expulsion along with increasing TOC and overpressure, and high expulsion value at Ro >1%. Source rocks with TOC and Ro <0.8%, type-II2 & III kerogen, overpressure <3Mpa, and low hydrocarbon expulsion volume are considered ineffective. Rocks with parameters between the two are considered effective. The high-quality class shows a strong empirical control on the distribution of tight oil in the Songliao Basin. This is followed by the effective source rock class. The ineffective class has no measurable contribution to the tight oil reserves. Because the hydrocarbon expulsion efficiency of source rocks is controlled by many factors, the lower limits of the evaluation parameters in different basins may vary. However, the classification method of tight source rocks proposed in this paper should be widely applicable.
•Hydrocarbon expulsion from lacustrine source rocks.•Conversion of kerogen to oil and importance of ensuing overpressure for hydrcarbon expulsion.•Quality grading of source rocks.•Source rock thickness distribution vis-à-vis tight reservoir charging potential.•Guide for tight oil and gas exploration and sweet spot prediction.
Detailed lithostratigraphic mapping of the Beekmantown Group of southwestern Quebec has refined the field application of the previously proposed tripartite division of the group (i.e., Theresa, ...Beauharnois, and Carillon formations). The group is a peritidal-dominated succession that accumulated on the epicontinental Laurentian passive margin. Biostratigraphic data based on conodonts from this group indicate an Early to early Middle Ordovician age and are partially time-correlative with the Wallace Creek to Naylor Ledge strata of the Philipsburg Group, southern Quebec. This conodont biostratigraphy sheds new light on the temporal evolution and depositional framework of the Beekmantown platform. The platform evolved as a distally steepened ramp during deposition of the Theresa Formation and the Ogdensburg Member of the Beauharnois Formation (early to middle Ibexian). Correlative strata of the Philipsburg Group include the Wallace Creek and Morgan Corner formations, which represent outer platform sediments. The coarse-grained sandstone of the Theresa Formation accumulated in the innermost platform, whereas coarse-grained carbonates of the Ogdensburg Member indicate open-marine, subtidal to intertidal carbonate sand shoals. By late Ibexian, the platform developed a pronounced margin where thrombolites flourished under high-energy conditions. These are represented by the thrombolite-rich Hasting Creek and Naylor Ledge formations of the Philipsburg Group. Consequently, a broad lagoon formed on the lee side of the platform margin, where low-energy conditions prevailed and accumulation of burrow-mottled dolostones of the Huntingdon Member of the upper Beauharnois Formation took place. The lagoon became more restricted during the latest stages of the basin fill (Whiterockian), and high intertidal to supratidal sediments of the Carillon Formation were deposited.
The Philipsburg tectonic slice is bounded to the west by a northeast-southwest-trending thrust fault (Logan's Line) and preserves 10 formations of Middle (?) to Late Cambrian (Milton, Rock River, and ...Strites Pond formations), Early Ordovician (Wallace Creek, Morgan Corner, Hastings Creek, and Naylor Ledge formations), and early Middle Ordovician (Luke Hill, Solomons Corner, and Corey formations) age. The strata were previously assigned to the Philipsburg Group. Early correlations between the Philipsburg succession and coeval strata of the St. Lawrence Platform were mainly based on sparse macrofauna and inferred stratigraphic position. Unconformities at the Cambrian-Ordovician and Early Ordovician-Middle Ordovician boundaries occurring in autochthonous St. Lawrence Platform and the allochthonous Philipsburg succession (Philipsburg tectonic slice) highlight new stratigraphic interpretations between the inner-shelf (St. Lawrence Platform) and the outer-shelf (Philipsburg) successions. The succession in the Philipsburg tectonic slice is divided into three new groups. The Middle (?) to Upper Cambrian Missisquoi Group (new) includes the Milton, Rock River, and Strites Pond formations. The upper boundary of the Missisquoi Group is defined by the upper unconformable contact between the Upper Cambrian Strites Pond Formation and overlying Lower Ordovician Wallace Creek Formation. The Missisquoi Group correlates with the Potsdam Group of the St. Lawrence Platform. The Lower Ordovician School House Hill Group (new) includes the Wallace Creek. Morgan Corner, Hastings Creek, and Naylor Ledge formations. The upper boundary of this group is marked by a regionally extensive unconformity at the top of the Naylor Ledge Formation and correlates with the younger Beekmantown-topping unconformity. The School House Hill Group is correlative with the lower to upper part of the Beekmantown Group (Theresa Formation and the Ogdensburg Member of the Beauharnois Formation) of the St. Lawrence Platform. The Middle Ordovician Fox Hill Group (new) consists of the Luke Hill, Solomons Corner, and Corey formations. This group correlates with the uppermost part of the Beekmantown Group (Huntingdon Member of the Beauharnois Formation and the Carillon Formation).
There are two unreconciled interpretations for the age and character of the boundary separating the Cambrian-Ordovician Potsdam and Beekmantown groups that underlie the Ottawa Embayment in eastern ...Ontario. These stratal groups consist of interior facies of the central Laurentian Platform. As exposed in the type section of the Nepean Formation (upper Potsdam Group), located in the City of Ottawa, the boundary was previously interpreted to be conformable and of Early Ordovician age. This intepretation was of enormous impact on subsequent regional geology compilations that showed a diachronous boundary across the platform interior. From recent subsurface analysis across eastern Ontario, the contact was interpreted to be disconformable, a sequence boundary separating Late Cambrian and Early Ordovician strata. This paper reexamines the type section. Lithologically, the group boundary should be repositioned downsection by ∼1.5 m. The contact now lies coincident with a disconformity that has a paleorelief of <10 cm. The proposed revision is geologically significant. Previous collections of Early Ordovician conodonts from the type section, used to define the age of what had been interpreted to be upper Potsdam strata, now fall entirely within the lower Beekmantown Group. Nepean (Potsdam) strata exposed in the type section remain undated. Regional correlation of the disconformity across the Laurentian platform suggests that Nepean strata at the type section are likely of Late Cambrian age. There now exists a regionally coherent separation of Cambrian and Ordovician sedimentation patterns in the Ottawa Embayment.
