Отправлено: 27.11.13 05:05. Заголовок: The low resistive Tertiary clastic reservoirs in KG Basin, India a challenge towards Hydrocarbon Exp

The low resistive Tertiary clastic reservoirs in KG Basin, India a challenge towards Hydrocarbon Explorations
Dr. K. Yadagiri*, A. D. Mohanty, IVSV Prasad

Summary
The Krishna-Godavari Basin, a passive continental margin located on the east coast of India having NE-SW structural trend.
The recent oil and gas strike in G2PA from Godavari Clay sequence in deep offshore has given impetus for present analyses.
The objective of the present paper is to address the cause for low resistivity of the pay sands within younger sequences like
Godavari Clay (Plio-Pleistocene) and Ravva (Mio-Pliocene). Evaluation of such low resistivity pay sands is a major challenge
as these are often overlooked owing to inherent limitations on the resolution of logging tools and unavoidable cut-offs in the
estimation of reservoir parameters.
Any hydrocarbon bearing layer with <3 ohm-m is called low resistivity pay.

Well cutting/core data in G2PA, G-1-A, G-4-B, G-4-C and other fields like A-1has indicated that the Godavari Clay sequence
is a massive, unconsolidated, mudstone / clay with incipient compaction and fine silty sand. The underlying formation i.e.
Ravva is a dominant sandy sequence with intervening shales.

Sedimentary features include hemi pelagic / colloidal clay nature, massive thick bedded mudstone/clay and oversized mudstone
rafts, mm scale parallel lamination and thin discrete sands in Godavari Clay and massive sandstones, mud rich conglomerates,
rhythmic occurrence of sand/mud rich layers, steeply dipping discordant bed forms and sharp bounding contacts are
diagnostic in Ravva Formation.

The sedimentary attributes collectively indicate muddy / sandy debris flow under dual flow sedimentary process in shelf - slope
depositional setting. Foraminiferal evidence in these wells has indicated outer shelf to bathyal environment. Log motifs furnish
mostly as sub marine channel sands, channel-levee complex.
Resistivity of these sands range between 1.5 and 4 ohm-m.
Matrix in these younger reservoirs is dominantly montmorillonite with subordinate mixed layers.

The causes for low resistivity in these sequences may be an inter play of multiple factors such as
-Bed thickness, the depositional system and its geologic age,
good presence of heavy mineral like pyrite,
being generated by marine diagenetic processes, presence of expandable clay mineral like montmorillonite and mixed layers and high formation water salinities.
Such shaly - sand reservoirs, a realistic multi -mineral combination would help assessing reservoir geology for optimizing
reservoir parameters in turn volumetric estimates.

As vast area in KG Basin is yet to be explored, predicting reservoir facies within the time frame tuned to the resolution of biochrons with seismic would definitely result in new hydrocarbon finds.
Keywords: Low resistivity anomaly, Godavari Clay, Deep water depositional system, Krishna-Godavari basin (KGB).

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