Changes

[[file:M114CH03FG03.jpg|300px|thumb|{{figure number|3}}Representative seismic sections with geological interpretation overlay illustrating the main aspects of basin structure. (A) North–south line A–A’. The shallow, inverted northern portion of the basin has never been buried as deeply as the south and is characterized by the presence of large, tilted fault blocks with very shallow [[Fatehgarh formation|Fatehgarh]] and [[Barmer Hill Formation]] reservoirs. By contrast, reservoirs in the southern part of the basin are at burial depths present day of ca. 3 km and have not been significantly inverted. The large, deep fault northwest of the [[Vandana]] area is a major hinge line, controlling deposition through time and a pivot point north across which there has been strong inversion. (B) West–east line B–B’ showing asymmetric half graben structure and the steep-sided nature of the Airfield High.]]

[[file:M114CH03FG03.jpg|300px|thumb|{{figure number|3}}Representative seismic sections with geological interpretation overlay illustrating the main aspects of basin structure. (A) North–south line A–A’. The shallow, inverted northern portion of the basin has never been buried as deeply as the south and is characterized by the presence of large, tilted fault blocks with very shallow [[Fatehgarh formation|Fatehgarh]] and [[Barmer Hill Formation]] reservoirs. By contrast, reservoirs in the southern part of the basin are at burial depths present day of ca. 3 km and have not been significantly inverted. The large, deep fault northwest of the [[Vandana]] area is a major hinge line, controlling deposition through time and a pivot point north across which there has been strong inversion. (B) West–east line B–B’ showing asymmetric half graben structure and the steep-sided nature of the Airfield High.]]

Structurally, the Barmer Basin is a typical intracontinental failed [[rift]], bounded by normal extensional faults that display both north-northwest–south-southeast and north-northeast–south-southwest orientations (Figures [[:file:M114CH03FG02.jpg|2]] and [[:file:M114CH03FG03.jpg|3]]). The basin is distinctly asymmetric in cross-section, with a regional dip to the east in the northern part of the basin, which flips to the west in the southern part. Fault-bounded [[horsts and ridges]] typically have a pronounced north–south elongation. Half-[[graben]] structures dominate, with deep [[synrift]] basins formed adjacent to [[footwall]] highs, the latter with evidence of crestal collapse and thinning of synrift stratigraphies onto the [[fault block]] highs indicative of significant initial rift topography. The presence of both hard- and soft linkages between [[fault]]s is indicative of a long history of fault evolution in the basin. Older, isolated north-northwest fault segments are linked via younger north-northeast striking faults to form an extensive, through-going fault system. Individual faults connected as a result of fault-tip propagation, overstep, and subsequent breach of the intervening relay ramps (Bladon et al., 2014<ref name=Bladonetal2014>Bladon, A. J., S. M. Clarke, S. D. Burley, N. J. Whiteley, V. Kothari, and P. Mohapatra, 2014, Structural inheritance in the Barmer Basin, India: Its influence on early-stage rift evolution and structural geometries: AAPG Annual Convention and Exhibition, Houston, Texas, [http://www.searchanddiscovery.com/documents/2014/10593bladon/ndx_bladon.pdf Search and Discovery Article #10593], accessed November 9, 2016.</ref>, 2015<ref name=Bladonetal2015a />, 2015<ref name=Bladonetal2015b />). The resulting fault network provides fluid migration pathway linkages across long distances and vertically across thick shale sequences between otherwise isolated carrier beds and reservoirs.

Structurally, the Barmer Basin is a typical intracontinental failed [[rift]], bounded by normal extensional faults that display both north-northwest–south-southeast and north-northeast–south-southwest orientations (Figures [[:file:M114CH03FG02.jpg|2]] and [[:file:M114CH03FG03.jpg|3]]). The basin is distinctly asymmetric in cross-section, with a regional dip to the east in the northern part of the basin, which flips to the west in the southern part. Fault-bounded [[horsts and ridges]] typically have a pronounced north–south elongation. Half-[[graben]] structures dominate, with deep [[synrift]] basins formed adjacent to [[footwall]] highs, the latter with evidence of crestal collapse and thinning of synrift stratigraphies onto the [[fault block]] highs indicative of significant initial rift topography. The presence of both hard- and soft linkages between [[fault]]s is indicative of a long history of fault evolution in the basin. Older, isolated north-northwest fault segments are linked via younger north-northeast striking faults to form an extensive, through-going fault system. Individual faults connected as a result of fault-tip propagation, overstep, and subsequent breach of the intervening relay ramps (Bladon et al., 2014<ref name=Bladonetal2014>Bladon, A. J., S. M. Clarke, S. D. Burley, N. J. Whiteley, V. Kothari, and P. Mohapatra, 2014, Structural inheritance in the Barmer Basin, India: Its influence on early-stage rift evolution and structural geometries: AAPG Annual Convention and Exhibition, Houston, Texas, [http://www.searchanddiscovery.com/documents/2014/10593bladon/ndx_bladon.pdf Search and Discovery Article #10593], accessed November 9, 2016.</ref>, 2015<ref name=Bladonetal2015a />, 2015<ref name=Bladonetal2015b />). The resulting fault network provides [[fluid migration]] pathway linkages across long distances and vertically across thick [[shale]] sequences between otherwise isolated carrier beds and reservoirs.

The basin is shallowest in the north with pre and synrift sequences exposed at the surface, whereas the deeper, southern part of the basin has continued to undergo burial to the present day (Figure 3). The shallow northern end of the basin is underlain by a large basement high where large rift blocks form traps such as those at the Mangala, Aishwariya, and Bhagyam fields (Figure 2), and the main reservoir formations occur at depths of less than ∼ 1 km (∼0.6 mi) present day (Figure 3). North of these fields’ top seals are commonly absent in the basin with the main reservoirs being close to or exposed at the surface and have been so since the Miocene. The post-Miocene burial history of this northern part of the basin is constrained from vitrinite reflectance (VR) and apatite fission track analysis (AFTA) studies and is known to involve widespread differential uplift, tilting, and erosion. Although this uplift was centered on the northern part of the basin, amounts of uplift vary between individual fault blocks, creating complex fault block geometries that appear to increase in throw amount to the western side of the northern basin, where up to ∼1200 m (∼4000 ft) of erosion are recorded. Uplift and tilting has been a major driver of spillage and re-migration of hydrocarbons with vertical faults providing fluid migration routes. The basin deepens dramatically to the south, and a more complete post-Miocene succession is preserved, although broad inversion structures are still developed. The rapid deepening to the south coincides with a faulted basement hinge zone that controls the younger regional tilting in the basin and may represent a cross-rift transform fault system.

The basin is shallowest in the north with [[prerift|pre]] and [[synrift]] sequences exposed at the surface, whereas the deeper, southern part of the basin has continued to undergo burial to the present day ([[:file:M114CH03FG03.jpg|Figure 3]]). The shallow northern end of the basin is underlain by a large [[basement]] high where large [[rift]] blocks form [[trap]]s such as those at the [[Mangala field|Mangala]], [[Aishwariya field|Aishwariya]], and [[Bhagyam field]]s ([[:file:M114CH03FG02.jpg|Figure 2]]), and the main [[reservoir]] [[formation]]s occur at depths of less than ∼ 1 km (∼0.6 mi) present day ([[:file:M114CH03FG03.jpg|Figure 3]]). North of these fields’ top [[seal]]s are commonly absent in the basin with the main reservoirs being close to or exposed at the surface and have been so since the [[Miocene]]. The post-Miocene burial history of this northern part of the basin is constrained from [[vitrinite reflectance]] (VR) and [[apatite fission track]] analysis (AFTA) studies and is known to involve widespread differential [[uplift]], [[tilting]], and [[erosion]]. Although this uplift was centered on the northern part of the basin, amounts of uplift vary between individual [[fault]] blocks, creating complex fault block geometries that appear to increase in throw amount to the western side of the northern basin, where up to ∼1200 m (∼4000 ft) of erosion are recorded. Uplift and tilting has been a major driver of spillage and re-migration of hydrocarbons with vertical faults providing [[fluid migration]] routes. The basin deepens dramatically to the south, and a more complete post-Miocene succession is preserved, although broad inversion structures are still developed. The rapid deepening to the south coincides with a faulted basement hinge zone that controls the younger regional tilting in the basin and may represent a [[cross-rift]] transform fault system.

Synrift depositional packages thicken into the hangingwalls of major synrift extensional faults. The juxtaposition of prerift and synrift reservoirs in the footwalls of major rift margin faults against thick synrift shales and source rock sediments of the hangingwalls confirms that fault activity was contemporaneous with deposition of the basin fill and enables direct, highly efficient migration of hydrocarbons from mature downthrown source rocks into carrier beds and traps.

[[Synrift]] [[deposition]]al packages thicken into the [[hanging wall]]s of major synrift extensional faults. The juxtaposition of [[prerift]] and synrift reservoirs in the [[footwall]]s of major rift margin [[fault]]s against thick synrift [[shale]]s and [[source rock]] [[sediment]]s of the hangingwalls confirms that fault activity was contemporaneous with [[deposition]] of the basin fill and enables direct, highly efficient [[hydrocarbon migration|migration]] of [[hydrocarbon]]s from mature downthrown source rocks into carrier beds and [[trap]]s.

Deep-seated extensional faults with a north-northwest–south-southeast orientation are rooted in the basement and extend upward to the early synrift sediment packages, providing potential fluid migration routes. These are often repeatedly reactivated by the north-northeast–south-southwest faults that extend through the synrift sequences, enabling early-generated hydrocarbons to extensively charge synrift sequences. However, these deep-seated faults do not cut the post-rift sequence, which remains largely isolated from the older sediment sequences.

Deep-seated extensional [[fault]]s with a north-northwest–south-southeast orientation are rooted in the basement and extend upward to the early [[synrift]] [[sediment]] packages, providing potential [[fluid migration]] routes. These are often repeatedly reactivated by the north-northeast–south-southwest faults that extend through the [[synrift]] sequences, enabling early-generated hydrocarbons to extensively charge synrift sequences. However, these deep-seated faults do not cut the [[post-rift]] sequence, which remains largely isolated from the older sediment sequences.

Strikingly different and much younger north–south-oriented faults occur along the western side of the basin (Figure 2) forming the linear Airfield High (Dolson et al., 2015), a structural feature bounded by steep faults that are predominantly strike-slip in nature (see Figure 2B). These faults extend further south into the basin where they connect with the Central Basin High. They are near vertical and in many cases can be seen in seismic sections to reach the surface, folding and faulting the youngest strata in the basin and creating flower-type structures. Although there is no surface expression of these faults, they are considered to have propagated southward in response to the ongoing Himalayan uplift and exhumation of the northern part of the basin. Locally, these steep faults have helped facilitate vertical hydrocarbon migration and charging of shallow structures.

Strikingly different and much younger north–south-oriented [[fault]]s occur along the western side of the basin ([[:file:M114CH03FG02.jpg|Figure 2]]) forming the linear [[Airfield High]] (Dolson et al., 2015<ref name=Dolsonetal2015 />), a structural feature bounded by steep faults that are predominantly [[strike-slip]] in nature (see Figure 2B). These faults extend further south into the basin where they connect with the Central Basin High. They are near vertical and in many cases can be seen in [[seismic section]]s to reach the surface, [[fold]]ing and faulting the youngest [[strata]] in the basin and creating flower-type structures. Although there is no surface expression of these faults, they are considered to have propagated southward in response to the ongoing [[Himalayan uplift]] and exhumation of the northern part of the basin. Locally, these steep faults have helped facilitate vertical [[hydrocarbon migration]] and charging of shallow structures.

 

==Stratigraphy==

==Stratigraphy==

The stratigraphy of the Barmer Basin is relatively simple as illustrated in Figure 4 (modified and updated from Dolson et al., 2015) and represents alternating cycles of deposition from fluvial and coaly swamps to deep water lakes into which fan deltas shed organic matter-rich debris flows and turbidites. Very minor marine influence at times of sea level high-stand is indicated by the presence of short-lived communities of marine foraminifera (Nummulites), dinocysts and ostracods (Sahni and Choudhary, 1972; Tripathi et al., 2009). The sedimentary sequence is dominantly Paleocene to Eocene in age and the basin fill overlies basement rocks of the Precambrian Malani Igneous Suite as well as prerift sediments of the Mesozoic fluvial Lathi and Ghaggar-Hakra formations (Compton, 2009; Beaumont et al., 2015; Bladon et al., 2015b). The Jurassic Lathi and Cretaceous Ghaggar-Hakra formations are sand-dominated, prerift reservoirs that are well exposed around the margins of the basin and occur along the northern uplifted outcrop limit. The Ghaggar-Hakra Formation is also encountered in wells in the main fields where they host significant hydrocarbon reserves. These sand-dominated sequences, together with the synrift Fatehgarh Formation, are extensive sheet-like alluvial deposits that form well-connected carrier beds and reservoirs (Figure 4). Lake margin fan deltas and deep water turbidite systems enclosed in the Barmer Hill Formation are more restricted in their extent and less well connected, but were deposited adjacent to source rocks and so are easily charged. Although there are multiple, thick, shale sequences within the Cenozoic basin fill that form regional seals, the most significant are those of the Sarovar Member of the Barmer Hill Formation, the Mandai Member of the Dharvi Dungar Formation, and the Juni Bali Member of the Akli Formation (Figure 4). All potential source rocks younger than the Dharvi Dungar Formation are thermally immature everywhere in the basin. The shallowest oil reservoirs are, however, younger than these source rocks and are encountered in the Thumbli Formation. These reservoirs require significant fault-assisted vertical migration to receive hydrocarbon charge. There are no accumulations above the Thumbli Formation anywhere in the basin, indicating that the overlying late Eocene Akli Formation is the topmost regional seal to the petroleum system across the basin that is only breached by a few deep-seated faults, many of which extend to the surface. A major depositional hiatus of late Oligocene to early Miocene age and subsequent tilting and uplift of the northern part of the basin in the Miocene completes the basin history.

The stratigraphy of the Barmer Basin is relatively simple as illustrated in [[:file:M114CH03FG04.jpg|Figure 4]] (modified and updated from Dolson et al., 2015<ref name=Dolsonetal2015 />) and represents alternating cycles of deposition from [[fluvial]] and coaly swamps to deep water lakes into which fan [[delta]]s shed organic matter-rich debris flows and [[turbidite]]s. Very minor marine influence at times of sea level [[highstand]] is indicated by the presence of short-lived communities of marine [[foraminifera]] ([[Nummulites]]), [[dinocysts]], and [[ostracods]] (Sahni and Choudhary, 1972<ref name=Sahniandchoudhary1972>Sahni, A., and N. K. Choudhary, 1972, Lower Eocene fishes from Barmer, Southwestern Rajasthan, India: Proceedings of the Indian National Science Academy, v. 38, p. 97–102.</ref>; Tripathi et al., 2009<ref name=Tripathietal2009>Tripathi, S. K. M., M. Kumar, and D. Srivastava, 2009, Palynology of Lower Paleogene (Thanetian-Ypresian) coastal deposits from the Barmer Basin (Akli Formation), Western Rajasthan, India: Palaeoenvironmental and palaeoclimatic implications: Geologica Acta, v. 7, p. 147–160.</ref>). The [[sediment]]ary sequence is dominantly [[Paleocene]] to [[Eocene]] in age and the basin fill overlies [[basement]] rocks of the [[Precambrian]] [[Malani Igneous Suite]] as well as [[prerift]] sediments of the [[Mesozoic]] [[fluvial]] [[Lathi formation|Lathi]] and [[Ghaggar-Hakra formation]]s (Compton, 2009<ref name=Compton2009 />; Beaumont et al., 2015<ref name=Beaumontetal2015>Beaumont, H., S. M. Clarke, S. D. Burley, A. Taylor, T. Gould, and P. Mohapatra, 2015, Deciphering tectonic controls on fluvial sedimentation within the Barmer Basin, India: The Lower Cretaceous Ghaggar-Hakra Formation: [http://www.searchanddiscovery.com/pdfz/documents/2015/51100beaumont/ndx_beaumont.pdf.html AAPG Search and Discovery Article #51100], accessed November 9, 2016.</ref>; Bladon et al., 2015b<ref name=Bladonetal2015b />). The [[Jurassic]] Lathi and [[Cretaceous]] Ghaggar-Hakra formations are sand-dominated, prerift reservoirs that are well exposed around the margins of the basin and occur along the northern uplifted outcrop limit. The Ghaggar-Hakra Formation is also encountered in wells in the main fields where they host significant [[hydrocarbon]] reserves. These sand-dominated sequences, together with the [[synrift]] [[Fatehgarh Formation]], are extensive sheet-like [[alluvial]] [[deposit]]s that form well-connected carrier beds and [[reservoirs]] ([[:file:M114CH03FG04.jpg|Figure 4]]). Lake margin fan [[delta]]s and deep water [[turbidite]] systems enclosed in the [[Barmer Hill Formation]] are more restricted in their extent and less well connected, but were deposited adjacent to [[source rock]]s and so are easily charged. Although there are multiple, thick, [[shale]] sequences within the [[Cenozoic]] basin fill that form regional [[seal]]s, the most significant are those of the [[Sarovar Member]] of the Barmer Hill Formation, the [[Mandai Member]] of the [[Dharvi Dungar Formation]], and the [[Juni Bali Member]] of the [[Akli Formation]] ([[:file:M114CH03FG04.jpg|Figure 4]]). All potential source rocks younger than the Dharvi Dungar Formation are thermally immature everywhere in the basin. The shallowest oil reservoirs are, however, younger than these source rocks and are encountered in the [[Thumbli Formation]]. These reservoirs require significant fault-assisted vertical migration to receive hydrocarbon charge. There are no accumulations above the Thumbli Formation anywhere in the basin, indicating that the overlying late [[Eocene]] Akli Formation is the topmost regional seal to the petroleum system across the basin that is only breached by a few deep-seated faults, many of which extend to the surface. A major depositional hiatus of late [[Oligocene]] to early [[Miocene]] age and subsequent [[tilt]]ing and [[uplift]] of the northern part of the basin in the Miocene completes the basin history.

 

Paleogeographic maps of the most important reservoirs and source rocks are shown in Figure 5. The most prolific reservoir is the late Cretaceous to early Paleocene synrift Fatehgarh Formation. The Fatehgarh Formation forms thick, sheet-like fluvial reservoirs that are best developed in the northern part of the basin (see Figure 5A). Fatehgarh Formation fluvial facies inter-digitate with basin margin alluvial fans of the Jogmaya Mandir Formation in the north and the Dhandlawas Formation in the center and south of the basin, where they are composed of lower-quality volcaniclastic sands particularly well developed around the Central Basin High. These Fatehgarh Formation reservoirs are directly overlain by the main source rock in the basin, the lacustrine shales of the Sarovar Member of the Barmer Hill Formation (Figure 5B). Migration from these source rocks into the Fatehgarh Formation reservoirs is therefore either downward or across fault juxtapositions and highly efficient. The fan delta and lacustrine turbidite sands of the Barmer Hill Formation are easily charged with hydrocarbons as they interdigitate with and are enclosed within the source rock. The Barmer Hill Formation also contains widespread diatomite deposits of the Bariyada Member (see Figure 4) that are intimately interbedded with the source rocks, forming a significant, laterally extensive but low permeability reservoir (Chowdhury et al., 2011).

Paleogeographic maps of the most important [[reservoir]]s and [[source rock]]s are shown in [[:file:M114CH03FG05.jpg|Figure 5]]). The most prolific reservoir is the late [[Cretaceous]] to early [[Paleocene]] [[synrift]] [[Fatehgarh Formation]]. The Fatehgarh Formation forms thick, sheet-like [[fluvial]] [[reservoir]]s that are best developed in the northern part of the basin (see [[:file:M114CH03FG05.jpg|Figure 5A]]). Fatehgarh Formation fluvial facies inter-digitate with basin margin [[alluvial]] [[fan]]s of the [[Jogmaya Mandir Formation]] in the north and the [[Dhandlawas Formation]] in the center and south of the basin, where they are composed of lower-quality [[volcaniclastic]] sands particularly well developed around the Central Basin High. These Fatehgarh Formation reservoirs are directly overlain by the main source rock in the basin, the [[lacustrine]] [[shales]] of the [[Sarovar Member]] of the [[Barmer Hill Formation]] ([[:file:M114CH03FG05.jpg|Figure 5B]]). [[Migration]] from these source rocks into the Fatehgarh Formation reservoirs is therefore either downward or across [[fault]] juxtapositions and highly efficient. The fan [[delta]] and lacustrine [[turbidite]] sands of the Barmer Hill Formation are easily charged with [[hydrocarbon]]s as they interdigitate with and are enclosed within the source rock. The Barmer Hill Formation also contains widespread [[diatomite]] deposits of the [[Bariyada Member]] (see [[:file:M114CH03FG05.jpg|Figure 4]]) that are intimately interbedded with the source rocks, forming a significant, laterally extensive but low permeability reservoir (Chowdhury et al., 2011<ref name=Chowdhuryetal2011>Chowdhury, M., M. Singhal, V. Sunder, T. O’Sullivan, P. A. Hansen, and S. D. Burley, 2011, Reservoir characterization of the low permeability siliceous Barmer Hill Formation, Barmer Basin, India: Society of Petroleum Engineers, v. SPE-146474-PP, p. 11–18.</ref>).

The overlying Dharvi Dungar Formation is a lacustrine, mud-dominated sequence that is also a significant source rock (Farrimond et al., 2015) and a major, thick regional seal across the basin, providing a widespread barrier to vertical hydrocarbon migration, only breached by young faults related to basin inversion and fault reactivation (Figure 4). Fluvial reservoirs formed during lake level low-stands occur in the Dharvi Dungar Formation at a variety of levels, most notably the Giral Member, but are productive only in the south of the basin (Figure 5C). These reservoirs are fine-grained, thin, and highly compartmentalized, and therefore difficult to charge with hydrocarbons except where local sourcing from their enclosing or underlying shales is possible. Productive deep-water lacustrine turbidites are also present in the Mandai and Kapurdi members of the Dharvi Dungar Formation. Fluvial reservoirs are locally charged on the Central Basin High in the shallower Thumbli Formation where basement-related faults propagate into the Eocene section.

The overlying [[Dharvi Dungar Formation]] is a [[lacustrine]], mud-dominated sequence that is also a significant [[source rock]] (Farrimond et al., 2015<ref name=Farrimondetal2015 />) and a major, thick regional [[sea]]l across the basin, providing a widespread barrier to vertical [[hydrocarbon migration]], only breached by young [[fault]]s related to basin inversion and fault reactivation ([[:file:M114CH03FG04.jpg|Figure 4]]). [[Fluvial]] reservoirs formed during lake level low-stands occur in the Dharvi Dungar Formation at a variety of levels, most notably the [[Giral Member]], but are productive only in the south of the basin ([[:file:M114CH03FG05.jpg|Figure 5C]]). These reservoirs are fine-grained, thin, and highly compartmentalized, and therefore difficult to charge with [[hydrocarbon]]s except where local sourcing from their enclosing or underlying [[shale]]s is possible. Productive deep-water [[lacustrine]] [[turbidite]]s are also present in the [[Mandai Member|Mandai]] and [[Kapurdi member]]s of the Dharvi Dungar Formation. Fluvial reservoirs are locally charged on the Central Basin High in the shallower [[Thumbli Formation]] where [[basement]]-related faults propagate into the [[Eocene]] section.

==See also==

==See also==

* [[Silurian Qusaiba shales]]

* [[Silurian Qusaiba shales]]

==References==

==References==