Chemostratigraphy and Identification of Fine-Grained Sedimentary Rocks Using Portable XRF
Portable x-ray florescence (PXRF) is a technique that is commonly used in hard rock mining and exploration. Lately, however, PXRF applications in oil/gas exploration and production are becoming more popular. While the stratigraphy of fine-grained sediments can be challenging, this paper presents an example of utilizing the PXRF technique for precise rock identification and chemostratigraphy during an active drill program.
Drill cuttings (221 discrete samples) were analyzed using a handheld Thermo Scientific Niton portable analyzer to report major elements (including light elements such as Mg, Al, Si, P, and S) and trace elements. The routine mud logging indicates 3 major intervals: (1) from 137 to 603 ft thick siltstone with interbedded sandstone and minor claystone, (2) from 603 to 723 ft thick sandstone interbedded with siltstone and local claystone, and (3) the interval from 723 to 2232 ft is dominated by claystone and siltstone interbedded with minor sandstone and rare limestone.
For rock identification, a ternary plot was created using Si, Al, and Ca which discriminates sandstone, siltstone, shale/claystone and limestone. This discrimination diagram can be used as a template for classification of common sedimentary rocks as well as tracking subtle vertical or horizontal changes in composition of fine-grained rocks.
In the studied well, the CaO/Al2O3 ratio indicates that up to 560 ft, the matrix relative to the rest of the well is more calcareous (as calcite and dolomite in the matrix) and after this depth the matrix becomes more clay rich. Calcium logs show that limestone only appears as a few thin intervals in deeper levels. Based on sulfur distribution in the log, three intervals are clearly visible: shallower than 785 ft with S < 2%, from 785–1085 ft with 3% > S > 2%, and deeper than 1085 ft with S > 3%. Positive correlation between S and Ba indicates that sulfur distribution is mainly controlled by barite. Vertical distribution of Ba shows that it can be used as a stratigraphic tool.
A silica versus Zr diagram shows both terrestrial and biogenic trends indicating that biogenic Si was produced locally in the claystone/shale interval mainly from 690 to 790 ft. It appears that this interval is the most brittle interval within claystone/shale unit. This is supported by high Si/Al ratio in this interval.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015