APPLICATION OF PALEOMAGNETIC TECHNIQUES TO SIDEWALL CORES: CHALLENGES AND METHODOLOGIC APPROACH

Abstract:

Paleomagnetism and rock magnetism have considerable value in hydrocarbon exploration and production although few subsurface paleomagnetic studies have been published so far. Paleomagnetic data that may be useful to the petroleum industry include magnetostratigraphy and dating of cores, susceptibility logging, magnetic fabric studies for paleocurrent analysis, use of remagnetization to date diagenetic events, and identification of thermal anomalies and tectonic rotations. In most of the cases, such paleomagnetic analyses require undisturbed rock samples that are not frequently collected in routine drilling operations. Indeed, the most common samples used for paleomagnetic studies comes from expensive vertical cores that usually are restricted to a narrow interval of interest. Contrastingly much less expensive, sidewall core is another type of sample collected from wells during logging operations just after the drilling phase. These samples used to be obtained by percussion with the disadvantage of providing mostly a fragmented and deformed material without orientation. The development of rotary sidewall coring system in more advanced logging tools allowed the acquisition of many undisturbed samples that can cover an extensive rock record through a well. In addition, with the advent of borehole imaging tools it possible to obtain azimuthal orientation of the holes after sidewall coring. Thus, a wide range sampling of intact and oriented rock samples made sidewall cores potentially useful for paleomagnetic studies. A test has been performed in an oil field in a pre-salt section of the Santos Basin, where several sidewall cores were collected in a development well. During preparation of this test, many challenges have been overcome, including the development of an adequate sampling protocol to retrieve oriented samples, adaptation of the actual laboratory paleomagnetic equipment to host samples of larger size, and the identification of potential sources of artificial remagnetization due to sampling procedures. In this study the application of non-destructive techniques (i.e. thermal demagnetization) was also tested aiming to find an effective compromise where any alteration of sample petrography is avoided, but the potential of paleomagnetic analysis is almost fully exploited. Applied for the first time in Brazil using this type of sample, the results show that it is possible to characterize the magnetic mineralogy of the target stratigraphic unit of a defined rock succession and retrieve reliable paleomagnetic directions that will be useful to outline the magnetostratigraphy. The developed protocol still has large room for further improvements, but the promissory results of this study already prove that paleomagnetic techniques can be useful and applied to the hydrocarbon exploration and production.