Log inversion method for nuclear log data of earth formations

摘要

Inverting nuclear log data for a geological formation surrounding a borehole involves acquiring nuclear log data for a borehole portion using a moveable nuclear logging tool and acquiring additional log data for the borehole portion using another logging device with superior resolution. Boundaries between adjacent zones are identified that exhibit an attribute of the geological formation to a detectably contrasting degree. From pre-acquired data describing one or more characteristics of the nuclear logging tool, a modeled log of the attributes is generated over the borehole portion, and a zone response is calculated from the pre-acquired data for each zone by using the boundaries to define an initial measure of the depth of each zone and ascribing a value of the attribute in dependence on the depth of each zone. The attribute of each zone is then calculated by deconvolving the nuclear log data using the zone response to minimize the difference between the nuclear log data and the convolution of the zone response and the attribute.

主权项

1. A method of generating an inverted nuclear log pertaining to a geological formation surrounding a borehole, the method comprising the steps of:
(a) acquiring nuclear log data in respect of at least part of the depth of the borehole using a moveable nuclear logging tool; (b) acquiring further log data, in respect of the said part of the depth of the borehole, using a further logging device, wherein the depth resolution of the further logging device is superior to that of the nuclear logging tool; (c) identifying from the further log data a plurality of boundaries between adjacent zones of the part of the depth that exhibit an attribute of the geological formation to a detectably contrasting degree; (d) generating from pre-acquired data describing one or more characteristics of the moveable nuclear logging tool a modeled log of the attributes over the said part of the depth; (e) calculating from the pre-acquired data a zone response in respect of each of the zones, using the boundaries to define an initial measure of the depth of each of the zones and ascribing a value of the attribute in dependence on the depth of each of the zones; and (f) calculating the attribute of each of the zones by deconvolving the nuclear log data using the zone response so as to minimize a difference between the nuclear log data and a convolution of the zone response and the corresponding calculated attribute.