(a) Discuss the direct and indirect geochemical methods used for prospecting of hydrocarbon deposits. (20 marks) (b) What are…

(a) Geochemical prospecting for hydrocarbons

Hydrocarbon geochemical prospecting operates on the principle that volatile light hydrocarbons migrate vertically via microseepage from deep subsurface reservoirs to the surface, creating both direct hydrocarbon concentrations and secondary physicochemical alterations in overlying soils and rocks.

Direct Geochemical Methods Direct methods measure upward-migrated light hydrocarbons (C₁–C₅) directly from soil, groundwater, or shallow sediments. Free soil gas analysis extracts interstitial vapors for headspace gas chromatographic analysis, and ratios such as C1/(C2+C3) distinguish thermogenic from biogenic gas. Direct indications also include visible oil and gas seeps and ultraviolet fluorescence of soil extracts for aromatic hydrocarbons. Acid extraction desorbs carbonate-bound or clay-adsorbed hydrocarbons from soil samples, yielding high-resolution anomalies of thermogenic gas. Microbial Prospecting for Oil and Gas (MPOG) detects anomalous populations of specialized hydrocarbon-oxidizing bacteria (such as Mycobacterium and Pseudomonas) that thrive on seeping alkanes. These techniques have been successfully applied across India in the Cambay Basin, Assam-Arakan fold belt, and the Krishna-Godavari (KG) onland areas by the Oil and Natural Gas Corporation (ONGC), whose KDMIPE laboratory at Dehradun runs surface geochemical surveys.

Indirect Geochemical Methods Hydrocarbon chimneys generate localized reducing environments that alter the redox potential (Eh) and pH of near-surface strata. Trace element halos develop as mobile elements precipitate under reduced conditions, creating characteristic iodine, vanadium, and uranium anomalies. Radiometric surveys detect potassium and uranium lows relative to the more immobile thorium, caused by leaching in the reducing seepage column, often forming a halo around the field. Other alteration indicators are bleaching of red beds, pedogenic calcite ('delta-C') anomalies and clay-mineral changes. Furthermore, microseepage-induced reduction of ferric iron produces diagenetic minerals like pyrite, greigite, and magnetite, which manifest as near-surface geomagnetic and electrical conductivity anomalies.

(b) Geobotanical Prospecting for Cu, Mn, and U

Plants growing over metalliferous substrates undergo diagnostic physiological and morphological changes driven by toxicity, altered enzyme activity, and nutrient competition.

For copper, excess metal induces iron deficiency, causing interveinal chlorosis, stunted shoot growth, leaf size reduction, and root truncation. The classic universal indicator is Aeolanthus biformifolius (the copper flower), alongside regional indicators like Becium homblei (now Ocimum centraliafricanum) in the Zambian–Congolese Copperbelt; in India, Polycarpaea corymbosa has been reported as a copper indicator over the Singhbhum copper belt. Manganese toxicity disrupts chlorophyll synthesis and activates destructive auxin oxidases, producing crinkling of leaves, marginal necrotic spotting, and stem deformation; the hyperaccumulator tree Macadamia neurophylla tolerates extreme manganese levels. Uranium deposits subject flora to heavy-metal toxicity and ionizing radiation, producing chromosomal mutations, morphological dwarfing, leaf chlorosis, and root malformations. The deep-rooting Astragalus pattersoni acts as an indirect indicator by hyperaccumulating associated selenium, while select aquatic bryophytes and lichens accumulate uranium directly.

Zinc Indicator Plants: Zinc toxicity restricts plant transpiration and induces severe dwarfing, chlorosis, and rosetting of leaves. Well-known global plant indicators for zinc include Viola calaminaria (the zinc violet) and Thlaspi caerulescens (now Noccaea caerulescens), which thrive exclusively on calamine-rich soils.

(c) Tonnage Factor and Reserve Calculation

The tonnage factor is the volume of ore required to yield one unit mass (expressed in m³/tonne or ft³/ton). Computed as the reciprocal of bulk density (TF = 1/Bulk Density), it accounts for the mineral specific gravity, structural porosity, and moisture content, serving as the fundamental scalar to transform geometric ore volume into tonnage (tonnage = volume ÷ TF = volume × bulk density). In SI units TF (m³/t) = 1/SG; in the imperial system TF (ft³/short ton) = 2000/(62.4 × SG).

Geometric Methods: Geometric estimation subdivides irregular ore zones into regular geometric prisms. The Cross-sectional (Trapezoidal) method calculates volume across parallel profiles separated by distance L via V = L/2(A₁ + A₂). The Polygonal method assigns each borehole a planar zone of influence via Voronoi polygons, multiplying area by borehole intersection thickness. The Triangular method constructs prisms between three adjacent drillholes (V = A/3Σ tᵢ). The Isopach method integrates areas between consecutive contour lines of equal thickness to compute slab volumes.

Graphic Methods: Graphic approaches determine surface boundaries directly from maps. The Planimeter method traces cross-sectional or isopach boundaries mechanically or digitally to measure true planar surface areas (A). The Graticule/Dot-Grid method superimposes a standardized dot or square grid over the orebody footprint, deriving area statistically from the count of intersection nodes enclosed by the boundary.

These classical methods are now supplemented by computer block models with inverse-distance and kriging (geostatistical) estimation, and results are reported as resources and reserves under the UNFC framework adopted in India's Mineral (Evidence of Mineral Contents) Rules, 2015.