Correlation of Sillimanite & Kaliophilite Minerals, TOC, Ro, and MBT from Drill Cutting of Well BS-03 in the Development of Shale Hydrocarbon, Brownshale Formation, Bengkalis Trough, Central Sumatra Basin, Indonesia

Sillimanite is a brittle mineral as a metamorphic mineral product which is generally derived from clay, along with an increase in pressure and high temperature (600˚C - 900˚C), and kaliophilite is also a brittle mineral as a potassium bearing in the sand-shale series, which contributes to the clay diagenesis process. In the development of shale hydrocarbon in the Brownshale formation in the Bengkalis Trough, Central Sumatra Basin, using the correlation of the XRD (bulk and clay oriented), TOC, Ro, and MBT analysis results from the drill cuttings of well BS-03, so that the fracable zone interval can be determined. From this correlation, it shows that the presence of sillimanite and kaliophilite minerals as minor minerals greatly affects the changes in shale character and hydrocarbon generation, where at depth intervals of 10,780 ft downward (sand series-shale) there is an interesting phenomenon, i.e. low MBT, low TOC, and high Ro, so it is believed that the depth interval of 10,780 ft downward is a fracable zone interval (brittle shale) which is a good candidate for hydraulic fracking planning, while the upper depth interval is a fracture barrier.


Introduction
Source rock in the Central Sumatra Basin, consists of four shale formations, namely: the Petani and Telisa Formation [1], Pematang Group [2], and coal Sihapas Group [3]. Based on geochemical analysis, it shows that only the Brownshale formation from Pematang Group is the main rock source in Central Sumatra, which is spread over several sub-basins (troughs), namely: Balam, Rangau, Kiri, Aman, and Bengkalis [4]. The depositional environment of this formation is formed from Lacustrine with lithological rocks consisting of laminated shales, brown in color, rich in organic matter, which indicates a depositional environment with calm water conditions [5].
From the results of previous research, it was stated that in general the Brownshale formation has good prospects for the development of shale hydrocarbon, supported by several parameters, including: TOC (fair -very good), kerogen type II/III, brittleness index greater than 0.48, and the rock compressive strength is below 70 MPa [5].
The brittleness index is the most widely used parameter to measure the brittleness of rocks [6]. In general brittleness is used as a descriptor in the selection of depth interval for hydraulic fracking planning, so brittleness is one of the most important rock mechanical properties, and is used in determining the prospect of shale hydrocarbon [7] [8] [9] [10] [11].
Mineralogical analysis using XRD (X-ray Diffraction) and MBT (Methyele Blue Test) from drill cuttings data can also be used to determine the type and character of shale [6] [8]- [13].
Sillimanite mineral is a brittle mineral which is a metamorphic mineral product that generally comes from clay, along with the increase in pressure and high temperature (600˚C -900˚C) with a burial depth of about 5 -6 km [14], and is very influential on hydrocarbon generation [15].
Kaliophilite is a mineral brittle, as a potassium-bearing mineral in the sand-shale series [16], contributing to the clay diagenesis process, which can change the character of shale from ductile to brittle, due to the process of changing the reactive smectite mineral to illite or kaolinite as non-reactive mineral [15].
The purpose of this research is to determine the fracable zone interval in the Brownshale formation by using the correlation of the results of XRD (bulk and clay oriented), TOC, Ro, and MBT analysis from drill cuttings of well BS-03, and from this correlation shows that the presence of sillimanite and kaliophilite minerals as minor minerals is very influential on changes in shale character and hydrocarbon generation which correlates with fracable zone interval as good candidates for hydraulic fracking planning.

Bengkalis Trough Study Area
The well BS-03 is the only well in the study area that penetrates the Brownshale formation of the Pematang Group, as the research target which is located on the

Geological Setting
The Central Sumatra Basin was formed during the Early Tertiary (Eocene-Oligocene) as a series of half grabens and horst blocks developed in response to an East-West direction of extensional regime [18].

Literature Review
The types of minerals contained in rock samples can be identified using XRD (X-Ray Diffraction) analysis with the bulk method [24], and the result is that the peaks can be read by the type of mineral based on the determinant peaks [25], and can be categorized into two, namely the major minerals and minor minerals.
From the results of the main minerals, namely Quartz, Clay, and Carbonate (Q-C-C), the brittleness index can be determined using the Jarvie Equation  The brittleness index is the most widely used parameter to measure the brittleness of rocks [6]. In general, brittleness is used as a descriptor in the selection of formation depth intervals for hydraulic fracking planning, so brittleness is one of the most important rock mechanical properties, and is used in determining the prospect of shale hydrocarbons [9] [10] [11].
Meanwhile, minor minerals include: Feldspar, Apatite, Pyrite, Dolomite, Sillimanite, Kaliophilite, etc. The presence of sillimanite and kaliophilite as minor minerals greatly influences the diagenesis process and the character of shale rocks.
Sillimanite is a brittle mineral as a metamorphic mineral product which generally derived from clay, along with increasing pressure and high temperature (600˚C -900˚C) with a burial depth of about 5 -6 km [14]. Kaliophilite is a brittle mineral, as a potassium bearing in the sand-shale series, which contributes to the clay diagenesis process [16]. Mondshine (1966) in his paper presented the shale classification based on MBT and X-ray diffraction (XRD) analysis, namely soft (ductile), firm (less ductile), hard (less brittle), brittle (brittle shale) as shown in Table 1. Babajide (2016) stated that the largest cation exchange rate is owned by allogeneic minerals (source rock fragment), while the smallest is owned by autogenic (chemical processes).  Maturity is the process of changing organic substances into hydrocarbons.
The maturity process is caused by an increase in temperature below the earth's surface. By knowing the maturity level of a source rock, it can be estimated that the ability of the rock to produce oil or natural gas. The level of maturity of a rock can be determined by Vitrinite Reflectance (Ro). Vitrinite Reflectance (Ro) values are expressed in percent (%).
In developing shale hydrocarbon commercially based on the results of previous research, several basic criteria are proposed as shown in Table 3.

XRD & MBT Analysis Using Drill Cuttings Data of Well BS-03
The results of semi-quantification calculations from XRD (bulk) analysis of 32 samples of drill cuttings to determine the percentage of minerals at each depth interval are shown in Table 4. From   [25], so they can be categorized as brittle minerals.
The result of the MBT analysis also shows an interesting phenomenon, namely at a depth interval of about 10,780 ft the value drops below 3 meq/100g, indicating the category of brittle shale [12].
Referring to the presence of the sillimanite and kaliophilite minerals (brittle minerals), as well as the low MBT value (brittle shale), then at the interval of 10,780 ft downward it is believed to be is a fracable zone interval (brittle shale) which is a good candidate for hydraulic fracking planning, while the upper depth interval is a fracture barrier.

Correlation of Lithofacies with XRD (Bulk), MBT, TOC, Ro Analysis of Drill Cuttings and Total Gas from Composite Log Data of Well BS-03
The correlation of lithofacies with the results of XRD (bulk), MBT, TOC, Ro analysis from drill cuttings, and total gas from composite log data of well BS-03 is shown in Figure 5, which shows a strong correlation with the depth interval in the sand-shale series. This can confirm the fracability model, i.e.: 900˚C), which supports the hydrocarbon generation process, and it is proven that at this depth interval Ro reaches a value of greater than 0.6%. b) The presence of kaliophilite, which is a brittle mineral, as a potassium-bearing mineral in the sand-shale series, contributes to the clay diagenesis process.
c) The value of MBT at the depth interval of the sand-shale series is generally low MBT, and has a strong correlation with low TOC and high Ro. This is in accordance with item a), where the deeper the maturity (Ro) is higher, so that the TOC value decreases. d) Sand-shale series interval is the most prospective Brownshale formation interval to produced hydrocarbon, based on items a), b), c), and the total gas from the composite log data of well BS-03.

Correlation of Lithofacies with XRD (Clay Oriented), MBT, TOC, Ro Analysis of Drill Cuttings and Total Gas from Composite Log Data of Well BS-03
From the correlation of lithofacies with the results of XRD (Clay Oriented) analysis, MBT, TOC, Ro from drill cuttings, and total gas from the composite log data of well BS-03 is shown in Figure 6, which shows the dominance of kaolinite and illite clay minerals compared to the other clay minerals, with the following explanation: 2) At the depth interval of the sand-shale series, there is sillimanite mineral, which is a brittle mineral as a result of alteration from clay at high temperatures (600˚C -900˚C), which supports the hydrocarbon generation process, and it is proven that at this depth interval Ro reaches a value of greater than 0.6%.
3) The presence of kaliophilite, which is a brittle mineral, as a potassium-bearing mineral in the sand-shale series, contributes to the clay diagenesis process, which 4) Sand-shale series interval is the most prospective Brownshale formation interval to produce hydrocarbon based on items 2, 3, and the total gas depth interval from the composite log data of well BS-03.

5)
Kaolinite and illite minerals dominate at depth intervals in the sand-shale series environment which is rich in potassium (K + ) mineral, as the product of clay diagenesis.