Geology 2022 Paper II 50 marks Describe

Q4

(a) What do you understand by sedimentary depositional environment ? Describe fluvial environment in detail. (20 marks) (b) Explain different processes of diagenesis in clastic sedimentary rocks. Describe common diagenetic structures. (15 marks) (c) Enumerate the sedimentary basins of India based on their petroleum prospects. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) अवसादी निक्षेपण वातावरण से आप क्या समझते हैं ? नदीय वातावरण का विवरण दीजिये । (20 अंक) (b) खंडज अवसादी शैलों में प्रसंधन की विविध प्रक्रियाओं को समझाइये । साथ ही विभिन्न प्रसंधननीय संरचनाओं का वर्णन कीजिये । (15 अंक) (c) पेट्रोलियम संभावनाओं के आधार पर भारत के अवसादी द्रोणियों का वर्णन कीजिये । (15 अंक)

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Directive word: Describe

This question asks you to describe. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'describe' demands detailed, structured exposition with visual support. Allocate approximately 40% of time/words to part (a) given its 20 marks, covering definition of depositional environment, fluvial system components (channel, floodplain, levee), and sub-environments (braided, meandering, anastomosing). Spend ~30% each on (b) and (c): for (b) explain compaction, cementation, dissolution, replacement, and authigenesis with structures like concretions and stylolites; for (c) enumerate basins by petroleum potential—Category I (Mumbai High, KG Basin), II (Cauvery, Assam-Arakan), III (Bengal, Vindhyan). Open with a brief integrative introduction on sedimentary geology's economic importance; conclude by linking diagenesis to reservoir quality and Indian energy security.

Key points expected

  • Part (a): Definition of sedimentary depositional environment (physical, chemical, biological parameters); detailed fluvial environment covering channel geometry, flow regime, sediment load, and vertical/lateral accretion deposits; distinction between braided (Siwalik-type), meandering (Gangetic plains), and anastomosing systems with Indian examples
  • Part (b): Five diagenetic processes in clastics—compaction, cementation (silica, calcite, iron oxides), dissolution (secondary porosity), replacement (calcite by dolomite), and authigenesis; diagenetic structures including concretions, nodules, stylolites, cone-in-cone, and septarian cracks with formation mechanisms
  • Part (c): Classification of Indian sedimentary basins by petroleum prospects—Category I (proven commercial: Mumbai High, Krishna-Godavari, Assam Shelf); Category II (identified prospects: Cauvery, Assam-Arakan fold belt, Rajasthan Basin); Category III (speculative: Bengal, Vindhyan, Himalayan foreland); mention ONGC classification and stratigraphic horizons (Cretaceous-Paleogene reservoirs)
  • Integration of Walther's Law for facies succession in fluvial systems; mention of sedimentary structures (cross-bedding, ripple marks, mud cracks) as environmental indicators
  • Link between diagenesis and reservoir quality: porosity-permeability evolution, cementation as porosity destroyer vs. dissolution as porosity creator; relevance to Cambay Basin and Mumbai High reservoirs
  • Economic significance: 65% of Indian petroleum from sedimentary basins; strategic importance of KG-D6, Rajasthan-Barmer discoveries; unconventional potential (shale gas in Damodar, KG basins)
  • Field evidence from Indian stratigraphy: Siwalik molasse as fluvial archive; Gondwana fluvial-lacustrine sequences; Proterozoic Vindhyan diagenetic features
  • Temporal framework: Phanerozoic petroleum systems (Mesozoic-Cenozoic productive intervals) vs. Proterozoic diagenetic overprints in Banded Iron Formations

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise definitions across all parts: for (a) correctly distinguishes alluvial fan, braided, meandering, and deltaic fluvial terminations with accurate hydraulic parameters (Froude number, flow regime); for (b) accurately orders diagenetic stages (eodiagenesis-mesodiagenesis-telodiagenesis) and correctly identifies cement types (syntaxial, poikilotopic); for (c) correctly applies ONGC's 1970s basin classification and identifies Cretaceous Deccan trap seal rocks in Mumbai HighBroadly correct concepts with minor errors: conflates braided and meandering channel deposits, misorders diagenetic stages, or lists basins without clear categorization by prospectivity; vague on cement mineralogyFundamental misconceptions: describes marine environment as fluvial, confuses diagenesis with metamorphism, or lists random geological features instead of sedimentary basins; no understanding of petroleum system elements
Diagram / cross-section20%10Three distinct, labeled diagrams: (a) fluvial system cross-section showing channel, point bar, levee, crevasse splay, floodplain with vertical succession; (b) diagenetic sequence diagram or photomicrograph-style sketch of cement relationships; (c) map of Indian sedimentary basins with petroleum categories color-coded; all with proper legends, scale, and stratigraphic annotationsOne or two adequate diagrams with partial labeling; generic river cross-section without fluvial sub-environment detail; simple basin location map without categorization; missing scale or legendNo diagrams, or crude unlabeled sketches; diagrams contradict text (e.g., meandering channel drawn as braided); illegible or irrelevant illustrations
Field evidence20%10Rich Indian field examples: for (a) cites Siwalik Group (Middle Siwalik braided, Upper Siwalik meandering), Sonia paleochannels, or Ganga-Yamuna interfluve studies; for (b) describes Bhander Limestone concretions, Kaimur sandstone cementation, or Barmer sandstone diagenesis; for (c) references specific fields (Bombay High, Mangala, Dhirubhai-1) with reservoir depth and lithologyGeneric field references without specificity; mentions 'Siwaliks' or 'Gondwanas' without stratigraphic precision; cites international examples (Mississippi, Niger Delta) without Indian correlationNo field evidence; purely theoretical treatment; invented or incorrect field locations; confuses stratigraphic units (e.g., Vindhyan as Gondwana)
Quantitative reasoning15%7.5Appropriate quantitative elements: for (a) mentions grain size ranges (sand: 0.062-2mm), flow velocities (braided: >2 m/s), or Siwalik sedimentation rates (~1mm/year); for (b) cites porosity reduction curves, cement volume percentages, or burial depth-temperature relationships for mesodiagenesis; for (c) provides recoverable reserve figures (Mumbai High: ~2.5 billion barrels, KG Basin: ~3 TCF gas)Sparse quantitative data: occasional grain size mention or depth ranges without context; round-number estimates without units; qualitative depth descriptors ('deep burial')No quantitative treatment; avoids all numerical data; incorrect orders of magnitude; confuses area (sq km) with volume or reserves
Indian / economic relevance20%10Strong integration of Indian energy security context: for (a) links fluvial reservoir architecture to groundwater aquifer management in Indo-Gangetic plains; for (b) emphasizes diagenetic controls on Cambay Basin Eocene reservoir quality; for (c) discusses strategic petroleum reserves, import dependence (~85%), recent discoveries (R-Cluster, Satellite fields), and unconventional potential with policy relevance (NELP, HELP regimes)Superficial economic mention: notes 'petroleum is important' or lists basins without strategic analysis; generic energy security statement without specific policy or reserve dataNo economic or Indian relevance; treats question as purely academic exercise; ignores petroleum prospectivity aspect entirely; outdated or incorrect economic data

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