Nannilam Fm
Type Locality and Naming
SUBSURFACE: The type section is located at exploratory well Nannilam-A (depth interval: 2425-1985) in the Nagapattinam subbasin. The hypostratotype is in the well Tirukkadaiyur-A (depth interval: 3490-3150 m). It was named after the Nannilam village by ONGC team steered by Venkatarengan et al., (1993) and issued as Document-VII by KDMIPE, ONGC, Dehradun (1993).
[Figure 1: Type-section Map of Subsurface Formations]
Lithology and Thickness
Sandy claystone. The formation comprises of alternations of shale/calcareous silty shale and calcareous sandstone. Shale is gray, fissile, hard with mica and carbonaceous matter admixed with microcrystalline calcite. The Sandstone is dirty white, soft and friable, fine to coarse and moderate to ill sorted, subangular to subrounded quartz and feldspars with occasional mica and inoceramus prisms and silt. It is inconsistently calcareous. Its thickness varies from 340-440 m. Part of Ariyalur Gr.
[Figure 2: Generalized Stratigraphy and Petroleum System of Cauvery Basin (from dghindia.gov.in)]
[Figure 3: Generalized Stratigraphy of Cauvery Basin (Provided by A. Nallapa Reddy & R. Nagendra)]
Relationships and Distribution
Lower contact
Unconformable with the Kudavasal Fm.
Upper contact
Conformable with the Portonovo Fm.
Regional extent
This formation is present in the entire basin except in the Nagapattinam subbasin.
GeoJSON
Fossils
Diagnostic planktonic foraminifera-Marginotruncana pseudolinneiana, M. coronata, Globotruncana linneiana, G. ventricosa, Rugoglobigerina rugosa.
Age
Depositional setting
Outer neritic-Upper bathyal.
Additional Information
It is partly age equivalent (biostratigraphic) to the Sillakkudi Fm in the outcrops