(a) Discuss the stratigraphic classification and depositional environment of Cuddapah Supergroup. Add a note on the tectonic…

(a) Cuddapah Supergroup: classification, environment and tectonic evolution (20 marks)

The Cuddapah Supergroup is a Proterozoic sedimentary and volcanic succession, up to about 12 km thick, in a crescent-shaped basin of about 44,000 km² in Andhra Pradesh, resting unconformably on the Archaean basement of the Dharwar craton.

Classification (Nagaraja Rao and others), from bottom to top:

• Papaghni Group: Gulcheru Quartzite (basal, with conglomerate) and Vempalle Formation (dolomite and limestone with stromatolites, basic sills and lavas).
• Chitravati Group: Pulivendla Quartzite, Tadpatri Formation (shale with tuff and sills) and Gandikota Quartzite.
• Nallamalai Group: Bairenkonda Quartzite, Cumbum Formation (shale, phyllite and dolomite) and Nagari Quartzite; the group is folded and thrust in the east.
• Kurnool Group (separated by an unconformity): Banganapalle Quartzite, Narji Limestone, Owk Shale, Paniam Quartzite, Koilkuntla Limestone and Nandyal Shale.

Depositional environment. The Gulcheru was laid down by braided rivers and in shallow water; the Vempalle in a shallow-marine peritidal carbonate setting, marked by stromatolites, with volcanism; the Pulivendla and Gandikota quartzites in shallow marine to tidal and fluvial conditions; the Tadpatri as shale, with volcanic ash, in a shelf environment; the Nallamalai as deeper marine, turbidite-type mud and sand; and the Kurnool on a stable shallow epicontinental shelf, with limestone and shale.

Tectonic evolution. The basin began as an intracratonic rift or pull-apart on the Dharwar craton, with basaltic magmatism, and then subsided thermally, building the Papaghni and Chitravati successions, with unconformities marking uplift. Compression in the east, related to the Eastern Ghats orogeny, thrust the Eastern Ghats belt westwards over the basin margin and folded the Nallamalai Group, while the west stayed nearly undeformed. The Kurnool Group was laid down later on a stable shelf.

(b) Classification of microfossils by composition and their use in hydrocarbon exploration (15 marks)

Classification by composition of the test or wall.

• Calcareous: foraminifera (benthic and planktonic), calcareous nannofossils (coccoliths), ostracods and calcareous algae.
• Siliceous: radiolaria, diatoms, silicoflagellates and sponge spicules.
• Phosphatic: conodonts, fish teeth and scales, and lingulid brachiopods.
• Organic-walled: palynomorphs (spores, pollen, dinoflagellate cysts, acritarchs), chitinozoa and scolecodonts.
• Agglutinated: foraminifera with tests of cemented sand grains, a further group.

Application in hydrocarbon exploration. Microfossils are small and abundant and survive in drill cuttings and cores, so they can be used at the well site.

• Biostratigraphy: they give the age of strata and correlate wells, and reveal missing sections from unconformities or faults.
• Palaeoenvironment: benthic foraminifera and ostracods give water depth and salinity, showing source, reservoir and seal facies and sequence-stratigraphic surfaces.
• Source-rock studies: palynofacies and kerogen types show source quality, and the colour of spores and conodonts (thermal alteration index, CAI) shows thermal maturity and the oil window.
• Well steering: microfossil markers guide the drilling of horizontal wells within the reservoir zone.

(c) Tunnelling in limestone terrain: problems and remedial measures (15 marks)

Problems specific to karst.

• Solution cavities and caves, empty, water-filled or clay-filled, at unpredictable places, giving roof collapse and irregular rock quality.
• Sudden inrush of water, or of mud and sand from filled cavities, under high hydrostatic pressure.
• Dewatering that alters groundwater flow, causing sinkholes and surface settlement, and drying of springs.
• Irregular, weathered rockhead and pinnacles, and joints widened by solution.
• Aggressive water attacking the lining, and possible gas in cavities.

Remedial and preventive measures.

• Thorough investigation by karst mapping, resistivity tomography, ground-penetrating radar, microgravity and seismic methods, with borehole probing.
• Probe drilling and geophysical forecasting ahead of the face, and pre-grouting with cement or chemical grout, or backfilling of cavities with concrete.
• Realigning the tunnel away from major karst or below the water table.
• Heavy support: steel ribs, shotcrete, rock bolts, forepoling or pipe-roof umbrellas, and face stabilisation; use of a closed-mode TBM with face pressure control.
• Drainage and pumping capacity, waterproof membranes and reinforced lining, monitoring of surface settlement, and emergency plans.