--> An Integrated Stochastic Modeling of Log and Core Data for Anomalous Log Responses; a Case Study, Tyagi, Anil K.; Dutta, Tanmoy; Abedeen, Ainul, #90100 (2009)

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An Integrated Stochastic Modeling of Log and Core Data for Anomalous Log Responses; a Case Study

Tyagi, Anil K.1
 Dutta, Tanmoy2
 Abedeen, Ainul2

1Logging & Petrophysics, Reliance Industries Limited, NaviMumbai, India.
2
Geology & Geophysics, Reliance Industries Limited,
Mumbai, India.

Cambay Basin, the north western part of Indian shield is varied from thick fluvial to deltaic deposits includes six tectonic blocks, the Patan, Mehsana, Ahmedabad, Tarapur, Broach, and Narmada, separated by faults aligned transverse to the general north-south axis of the rift. The basin is known for its organic rich black shale, generally known as Cambay shale. A well drilled in the study area has encountered an anomalous zone in the Cambay shale showing very exceptional log signatures. The prediction of lithology and presence of hydrocarbon become very doubtful because of the kind of log signature and failure of formation tester.

Very high resistivity and low density indicates the presence of hydrocarbon which in turn discarded by the high neutron .High neutron itself indicative of organic matter while some non-conductive mineral with thermal neutron absorber elements also cause the high resistivity and high neutron. Different log curves responses differently to various factors like presence of mineral, organic matter and dead oil. To eliminate the uncertainty, XRD of the two core samples was carried out, which confirms a greater amount of chamosite, a low CEC Fe-rich chlorite in conjunction with organic matter filled up in the larger pores.

To quantify the presence of minerals and the organic matter, a stochastic analysis was carried out with the help of core data. The analysis is done by optimizing simultaneous equations described by the interpretation model supported by core XRD data. It is based on the inversion principle in which log measurements of tools and response parameters are used together in response equations to compute volumetric results for formation components. The analysis also uses forward modeling also called log reconstruction to compute tool response. A log reconstruction problem is computed for each inverse problem. The reconstructed logs are compared against input data to determine the quality of volumetric results from the inverse problem. The challenge in such analysis is to make a model which is geologically and mathematically feasible.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil