The Pematang Group is the main source rock of hydrocarbon in the Central Sumatra Basin and is the oldest sedimentary layer in Paleogene (Figure 3). Syn-rift sediment the formation of Pematang Group is deposited unconformity in the half-graben.

The discovery of fossils of ostracods, freshwater gastropods, spores, pollen, dinoflagellates, algae and fern debris on core rock samples and drill cuttings powder in all major troughs and the absence of foraminifera provides an indication of the non-marine depositional environment in humid and tropical environments [5]. Based on its lithological characteristics, the Pematang Group is divided into three formations, namely: Lower Red Bed Formation, Brown Shale Formation and Upper Red Bed Formation.

1) Lower Red Bed Formation

This formation consists of claystone, siltstone, arctic sandstones and a few conglomerates deposited in the environment of alluvial plains and in fluvial environments. The lower part of this formation in several deep basins can reach a thickness of 3000 meters. Sandstones in this formation have poor quality as a reservoir because they are still very close to the source and have poor sorting.

2) Brown Shale Formation

As the name implies, this formation consists of brown shale and deposited on the Lower Red Bed Formation with a lacustrine depositional environment. Shale in this formation is rich in organic matter, has a fairly good laminate which indicates that the shale is deposited in fairly calm water conditions. This shale is rich in organic matter so this formation is a hydrocarbon source rock for the reservoir located in the Central Sumatra Basin. This formation is also composed of delta and turbidite fan deposits. Turbidite deposits formed by granular flow mechanisms have been used as exploration targets which generally have stratigraphic trap types.

3) Upper Red Bed Formation

This formation is deposited in the final stage of F1 phase tectonics. Increasing the speed of sedimentation and clastic supply causes the basin to become full and the environment to become fluvial and alluvial. The lithology of this formation is in the form of red-green sandstones, conglomerates and claystones. Sandstone in this formation is an exploration target.

X-Ray Diffraction (XRD) analysis is conducted to identify the types of minerals contained in each rock sample using the bulk analysis method. In bulk analysis, shootings are carried out with shooting angles of 3˚ up to 90˚. After getting the results of shooting or running samples using the XRD tool, the results of the analysis are presented in the form of peaks from the XRD reading chart and the types of minerals present in the sample can be determined based on determinant peaks [6].

The type of mineral that has been identified is carried out a semi-quantification analysis to determine the percent of minerals in each depth sample. The mineral percentage is calculated using Equation (1).

Percentage   of   minerals   A = I A ( I A + I B + ⋅ ⋅ ⋅ + I n ) × 100 % (1)

where I is the determinant peak intensity of one type of mineral, while A, B, ∙∙∙ n are the types of minerals identified in each sample.

Brittleness Analysis from the XRD Laboratory Test

Brittleness is the measurement of stored energy before failure, and is function of rock strength, lithology, texture, effective stress, temperature, fluid type, diagenesis, and TOC. Brittleness Index (BI) is the most widely used parameter for the quantification of rock brittleness [7].

In recent years, brittleness has been used as a descriptor in selecting formation zone intervals for hydraulic fracking ( [8] [9] [10] ). Therefore, brittleness is one of the most important rock-mechanical properties, and is used in determining prospecting of shale hydrocarbon.

Brittleness index based on the results of XRD analysis can be calculated using a formula from Equation (2) [8] as follows:

B I ( Jarvie ,   2007 ) = W q t z / W t (2)

where W_qtz = weight of quartz; and W_t = total mineral weight

Methylene Blue Test (MBT) is used to determine the ability of clay to bind the cation of a solution, namely by using methylene blue to measure the total cation exchange capacity of the clay, where the cation exchange depends on the type and crystallinity of the mineral, the pH of the solution, the type of cation exchanged, and concentration of mineral content contained in clay.

The cation exchange capacity (CEC) is based on the sequence of ionic bond strengths as follows:

Li⁺ < Na⁺ < H⁺ < K⁺ < NH⁴⁺ < Mg³⁺ < Ca²⁺ < Al³⁺

The largest cation exchange value is owned by allogenic minerals (fragments of source rock), while the smallest is owned by authogenic (chemical processes). The cation exchange capacity of several types of clay minerals is shown in Table 1.

Determination of the total original organic carbon (TOC_o) from source rock provides a quantitative means for estimating the total volume of hydrocarbons that can be produced depending on the type of kerogen. Broadly explored areas generally have mature source rocks, so it is not easy to determine the original values. Consideration of the TOC component helps in understanding how to restore highly mature TOC to TOC_o [8].

Rocks that contain a lot of carbon are called high source rocks containing carbon elements (high TOC). The requirements as a source rock, which contains high organic content and has a type of kerogen that has the potential to produce hydrocarbons and has reached a certain maturity, so that it can produce hydrocarbons. Peters and Cassa (1994) divide into 5 types of source rock, as shown in Table 2.

Shale hydrocarbon parameters that can be produced commercially include several criteria, as follows [13]:

1) TOC: > 2%

2) Shale thickness: >100 ft

3) Moderate clay content: <40%

4) Brittle Index shale: >0.48

5) Brittle shale (fracability): i.e. low Poisson’s ratio & high Modulus Young

In the development of shale hydrocarbon in the Brownshale formation at Bengkalis Trough based on XRD (bulk) and MBT analysis from drill cuttings data, using the drill cuttings of well that penetrated the Pematang formation, namely well BS-02, well BS-03, well KCBS-01, and well KRBS-03. Figure 4 shows the correlation of well logs from wells BS-02, BS-03, KCBS-01 and KRBS-03. Based on the correlation of the four wells, only well BS-03 penetrated the Brownshale formation, which was confirmed by composite log data such as seen in Figure 4, so the mineralogy (XRD) and MBT analysis to determine the fracability model of Brownshale formation only used the drill cuttings of well BS-03 data with depth interval of 9860 to 11,642 ft which were divided into 10 ftMD intervals of 46 samples. Dominant peaks in the graph are calculated basal spacing values (d) to be analyzed for mineral types by matching these values with the mineral standards in the “Handbook of Mineralogy”.

The results of the XRD analysis (bulk) of the Brownshale Formation drill cuttings sample in well BS-03, which include percent of Quartz, Clay and Calcite content versus depth, are shown in Figure 5.

From the results of the bulk analysis at each depth interval, it is obtained the main mineral content percent (Quartz, Clay, and Calcite), so that by using equation 2 a Brittleness index can be determined, as shown in Figure 6. Shale hydrocarbon parameters that can be produced commercially one the criteria is brittleness index > 0.48 [13], and in Figure 5 shows that the entire depth interval of the Brittleness index value is greater than 0.48, so it can be concluded that the Brownshale formation from the results of mineralogical analysis is brittle.

From the results of the bulk analysis, in addition to the main mineral content, there is an interesting phenomenon, namely the presence of minerals sillimanite and kaliophilite significantly starting at a depth of 10,780 ft towards the bottom of the Brownshale formation, where both minerals have tenacity: brittle [6]. Figure 7 shows the presence of the sillimanite and kaliophilite minerals.

In determining the prospect interval (sweetspot fracable window) of shale

hydrocarbon, not only the brittleness index is calculated from the XRD (bulk) analysis, but also from the results of the MBT analysis. From the results of the MBT analysis can be seen an interesting phenomenon, namely at a depth interval of about 10,780 MDft the value drops below 3 meq/100 grams, including the brittle category [14]. Figure 8 shows the results of the MBT analysis of drill cuttings of well BS-03.

Referring to the presence of sillimanite and kaliophilite minerals, as well as low MBT values, at a depth interval of 10,780 ft MD to the bottom depth interval it is believed to be a hydraulic fracking candidate. MBT values, at these intervals

generally also low values, including the brittle category [14].

From other calculation methods, it can also be determined the nature of brittle rocks using ternary diagrams based on their mineral content, namely quartz, clay and calcite. Based on the XRD analysis (bulk) results of mineral compositions from the drill cuttings, a plot can be made on the ternary diagram. The ternary diagram plot of the XRD analysis (bulk) results of drill cuttings sample of the Brownshale formation of Well BS-03 is shown in Figure 9.

From the Figure 9 it can be seen that the XRD results (bulk analysis) from the Brownshale drill cuttings are distributed between zone 1 to zone 3, where the spread of zone 3 is more dominant. This can be discussed, as follows:

· Zone 1: Quartz dominant - Clay & Carbonate minor (Brittle quartz rich)

· Zone 2: Carbonate dominant - Quartz & Clay minor (Brittle carbonate rich)

· Zone 3: Quartz & Carbonate Balance - Clay minor (Ductile, hard to frac)

The results of the TOC analysis of the Brownshale drill cuttings of well BS-03 were performed laboratory tests using 10 drill cuttings samples shown in Table 3.

In the development of shale hydrocarbon in the Brownshale formation from the results of mineralogy (XRD), MBT and TOC analysis of the drill cuttings sample

of well BS-03, it can be integrated to determine a good sweetspot interval as a hydraulic fracking candidate. Figure 10 shows the integration of the results of mineralogy and TOC analysis of Brownshale drill cuttings.

From Figure 10 which is the correlation of depth (ft), mineral content, brittleness index, MBT, and TOC, that the Brownshle formation can be determined as a sweetspot fracable interval as a good hydraulic fracking candidate with the following criteria [13]:

1) TOC: 1475 wt% Avg (>1 wt%)

2) The total thickness of the Brownshale formation: 1028 ft (> 100 ft).

3) Moderate clay content: 10.57% Avg (<40%)

4) Brittless index: 0.79 Avg (> 0.48)

5) High Fracability: MBT: <3 cc meq/100gr (brittle), >3 cc meq/100gr (hard/less brittle).

S1 = Free Hydrocarbons; Oil Production Index = Transformation Ratio = S1/(S1 + S2). *TOC content by Leco Analyzer.

From the correlation in Figure 9, it can also be seen that at the bottom of the depth interval as a fracable sweetspot window, and at the upper depth interval of the Brownshale formation, it is believed to be a fracture barrier.