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The Mannar Sub-basin forms the southern section of the main Cauvery Basin, lying between western Sri Lanka and the southeastern Indian coastline. Water depths range from 50 – 3,000 m, and the basin contains over six seconds of (?) Late Jurassic/Early Cretaceous to Recent sediments.
Geohistory
The Mannar Sub-basin developed during at least two periods of rifting and associated continental break-up, as part of the multi-phase break-up of Gondwana during the Mesozoic. The first phase began as a precursor to the commencement of sea floor spreading in the Bay of Bengal, and was followed by a second phase of rifting associated with the detachment of Madagascar from the western side of the developing Indian sub-continent. This was followed by a prolonged period of thermally driven subsidence that was punctuated by discrete episodes of uplift and erosion. Consistent with the tectonic setting of the region, four discrete tectono-stratigraphic packages have been recognised.
The oldest package, Megasequence 1 was deposited during the initial syn-rift phase of basin development, prior to the commencement of sea floor spreading in the Bay of Bengal, west of Sri Lanka. Megasequence 2 sediments were deposited during the late rift and sag phase, after the commencement of sea floor spreading in the Bay of Bengal, but before the onset of spreading about the West Indian Ridge. Megasequence 3 was deposited during a Tertiary sag phase of basin development, which terminated in the Middle Miocene following a compressive event. Subsequent basin-wide regression resulted in deposition of Megasequence 4.
Although there is a lack of direct geological control, seismic data indicates the presence of 1-2 seconds thickness of Megasequence 1 sediments. Deposited within the developing grabens, Early Cretaceous and older, syn-rift sediments are anticipated to exhibit strong palaeo-control with significant local sediment thickness variations and distributions confined to deeper structural settings. During the deposition of Megasequence 2 basement is predicted to have undergone relatively rapid subsidence involving both sag and active tensional downfaulting. In the Mannar Sub-basin this phase was accompanied by basaltic volcanism. The presence of a number of distinctive units, in some cases separated by unconformities and comprising distinctive lithologies, presumably reflects upon a complex structural and depositional history at this time, specific periods being punctuated by rapid subsidence, or quiescence and/or uplift. This depositional package was terminated at the end of the Cretaceous by strong uplift and erosion. In Sri Lankan waters, the thicknesses of Megasequence 2 sediments reach around 750 m in the Mannar 1-1A Well. The sedimentary succession demonstrates a primarily transgressive nature from continental and marginal marine sediments in the Cenomanian and Turonian, through inner to deep outer shelf sandstones and siltstones in the Coniacian to Santonian, to more shelf and deep water basinal shales and alternating sandstones in the Campanian. During the Maastrichtian, shelf to deep marine environments prevailed, but were deposited in a more regressive regime. Some of the deep water sediments were deposited within turbidite systems.
The onset of Megasequence 3 was marked by renewed marine transgression in the Paleocene. Shelf and deep water depositional systems became widespread. Turbidite, slump and debris flows characterised the slope and basin floor, and at least three major floor and slope fan deposition episodes occurred, in the Paleocene, Eocene and Early Miocene, which resulted in extensive stacked fan floor and slope deposits. A period of Late Paleocene carbonate development occurred along the basin margin, suggesting a period of relative structural quiescence prior to local uplift, unconformity development and influx of clastics during the Early Eocene. Further carbonate development took place during the Early Oligocene and Early Miocene, with reefs developing on highs and areas away from the main sediment influxes.
Deposition of Megasequence 4 took place from the Late Miocene onwards in a predominantly regressive regime. Enhanced hinterland uplift and clastic supply caused shelf systems to prograde westwards from the margin, drowning the isolated carbonate systems. Uplift and progradation resulted in outershelf and slope instability, causing redeposition of earlier sediments onto the lower slope and basin floor as debris flows and turbidites.
Source Rocks
On the basis of sediment character, geochemical analysis, comparison with intervals in the northern Cauvery Basin and maturity, the Lower Cretaceous, and probably underlying Jurassic, sections appear to contain the best potential hydrocarbon source rocks.
Based on comparisons with proven intervals in the northern Cauvery Basin, there are two potential source intervals, the first associated with the basal transgression over the Albian unconformity (base of Megasequence 2), the second in the Late Cretaceous open marine sediments. Recent seismic data in the Mannar Sub-basin shows the presence of deep sedimentary fill, where sedimentary thicknesses are in excess of 5,000 m. In such localities, adequately mature source rocks containing oil-prone kerogen with a high hydrocarbon generating potential within the deeper (?)Late Jurassic to Early Cretaceous Megasequence 1 sediments may exist.
Synthetic burial history modelling suggests that the Megasequence 1 section is within the oil and gas maturity windows, with peak generation having occurred in the Late Cretaceous and also the Late Tertiary. Multiphase generation has occurred as a result of multiple heat pulses and the deposition of thick post break-up sediments.
Reservoirs
With limited well control on the syn-rift Megasequence 1 it is difficult to assess the reservoir potential of these units. Fundamental principles of syn-rift sedimentary fill suggest that sediments may be terrestrial and coarse grained, fining upwards with increasing depositional maturity. Proven reservoirs exist in the Late Cretaceous sediments of Megasequence 2 in the northern Cauvery Basin (e.g. PH-9-1). In the Mannar Sub-basin, Megasequence 2 sediments are thought to contain some good clastic reservoir intervals. Cretaceous age basal sandstone units of this megasequence with excellent reservoir characteristics are common in the basin. The Pearl 1 Well encountered a gross Cretaceous sandstone thickness of 800 m, with porosities exceeding 20%. Many of the reservoir units are interpreted as turbidites, with the best development occurring off the shelf in the Mannar Sub-basin. Thick mounding of sediments is observed on several seismic lines within the basin. Prominent relief (horst and graben rift architecture) at this time probably restricted lateral distribution of these packages, with many aprons and fans being locally restricted around the flanks of basement highs. Porosities in these units range between 15 and 30%. Better sorting and more extensive turbidite deposition probably occurred towards the end of the Cretaceous as the incipient relief was infilled.
In the northern Cauvery Basin, Tertiary sandstone reservoirs of Megasequence 3 are well established, and often include turbidites. In the Mannar Sub-basin, turbidite and chaotic slump and debris flows on the lower slope and basin floor have been identified on recent seismic. Three periods of major floor and slope fan deposition during the Paleocene, Eocene and Early Miocene have provided stacked fan floor and slope deposits, resulting in a major fairway along the basin floor and lower slope of the Sri Lankan margin. Channelling in these units is more apparent in the younger units, reflecting more basinward distribution across the distal portions of the slope as gradients increased and sediment by-pass of the shelf became more extensive. This is likely to have become more marked during significant sea level falls which were experienced from the Early Eocene and particularly during the Early Oligocene and Early Miocene. Porosities in these sandstones generally increase higher in the section as a result of reduced diagenetic and compactional factors, with porosities of 20 – 40% encountered in Early Miocene units. Carbonate reservoir potential is also present, especially in the stratigraphically higher levels of Megasequence 3. Good porosities (>20%) are reported for these limestones, particularly in the Early Oligocene and Early Miocene.
Although similar deposition and significant slope and outer shelf by-pass continued during Late Miocene to Recent times in Megasequence 4, there is not considered to be any major reservoir units present.
Traps
Several potential trapping mechanisms are suggested to be present in the Mannar Sub-basin, both by analogy to those occurring in the northern part of the Cauvery Bain, and as evident on seismic sections. Potential Late Jurassic and Cretaceous age horst and tilted fault-block traps are recognised in the syn-rift and early sag phase succession (megasequences 1 and 2). These fault-blocks also created relief, over which sediments of Megasequence 2 and particularly the early part of Megasequence 3 were deposited and formed compactional drape anticlines. Late Miocene compressional tectonism is not as pervasive in the Mannar Sub-basin as it is in the northern Cauvery Basin, but transpression related structures may form potential traps. Clastic stratigraphic traps are likely to occur in the Late Cretaceous to Early Miocene turbidite systems. On the slope and basin floor, fans, mounds, channels and ponded turbidites are observed at breaks in gradient. The development of carbonate stratigraphic traps may have occurred during periods of reef development in the Eocene and Late Oligocene. Development of these is fairly restricted, to basin highs and areas remote from clastic influxes. |