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 north side depocenter of the Bengkalis Trough located in Riau Province, Indonesia (Figure 1). Currently the Bengkalis Trough area is managed by two oil company operators, namely the Malacca Strait EMP Group and the CPP Block BOB PT. Bumi Siak Pusako-Pertamina Hulu, which each gave permission and contribution to conduct this research.

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]. A divergent transform boundary (non-coupling) between the Sunda Microplate and the Indian Oceanic Plate during Paleogene gave rise to extensional regime and crustal stretching of the western part of the Sunda Land resulting in the formation of Pematang type grabens [19]. Pematang Graben Development can be divided into 4 stages:

· Stage I Pre-Graben (Early Eocene)

During the early Eocene the Indian Ocean Plate was moving N 10˚E [19]. At this time approximate north-south to north-west-southeast lines of weakness with complementary northeast-southwest shears developed. These lines of weakness later became the hinge lines and fault scarps of graben and half-graben structures. During the early to Middle Eocene, the angle of plate convergence increased from N 10˚E to N 50˚E with a resulting minor compressional component [20]. Gentle crustal doming resulting from subduction also began at this time compensating the weak compressional stress, especially in tensional stress system. Incipient block rotation occurred along the earlier developed lines of N-S weakness. Deposition of Lower Red Beds Formation began in developing shallow graben.

· Stage II Graben (Middle Eocene)

Rapid graben development began as a result of relaxation of the Middle Eocene compression by a change in the plate convergence angle to the present N 20˚E. Deposition of the Brown Shale and Coal Zone Formations occurred during this stage of graben development.

· Stage III Pematang Structuring (Oligocene)

During the late Early Oligocene continued spreading of the graben and increased episodic right-lateral wrench movement occurred resulting in Pematang Structuring.

· Stage IV Lake Fill (Late Oligocene-Early Miocene)

This stage represents the beginning of a tectonic phase resulting in rapid deposition of the Lake Fill Formation. It is characterized by uplift and rapid erosion of highland areas, and culminated with the major unconformity at the end of Pematang deposition. Regional subsidence in the Early Miocene accompanied by a major marine incursion, ended the rift phase of graben development.

The Pematang Group Brownshale Formation is the main source rock of hydrocarbons in the Central Sumatra Basin and is the oldest sedimentary rock aged Eocene to Oligocene which is deposited unconformity in the half-graben [4] [21].

The stratigraphic analysis can be determined by the sequence and distribution of the Brownshale zone. Based on the correlation between wells, it is the most reliable method in stratigraphic analysis, but it cannot be done, because only the well BS-03 penetrates the Brownshale zone. Determination of marker names using tectonic episodes using references from Lambiase (1990), as shown in Figure 2,

namely: Rift Initiation, Early Syn Rift, Mid Syn Rift and Late Syn Rift. The naming is considered to be more representative of the tectonostratigraphy of the Central Sumatra Basin because it reflects the tectonic and sedimentation processes produced in each episode.

Rift Initiation is the beginning of the formation of rifting, where the initials of new faults are formed to form fluvial deposits which are dominated by sandstone lithology to conglomerates, this can be analogous to the Lower Red Bed Formation. Early Syn Rift is characterized by active large faults and rapid subsidence, resulting in a lacustrine environment and depositing fine-grained materials such as claystone and siltstone which are equivalent to the Brownshale Formation (Lacustrine Brownshale). Middle Syn Rift is the initial phase of decreased subsidence activity and regional silting occurs causing deltaic to fluvial deposits to form, indicating the final phase of the Browshale Formation (Fluvio-Deltaic Brownshale). Late Syn Rift is the final phase of subsidence activity where a fluvial to deltaic environment is formed which is dominated by sandstone lithology which reflects the Upper Red Bed Formation.

In tectonostratigraphy, the basin filling deposits in the Central Sumatra Basin consist of 4 stages, namely: Rift Initiation (Lower Red Bed), Early Syn rift (Lacustrine Brownshale), Mid Syn Rift and Late Syn Rift (Upper Red Bed). The main targets for hydrocarbon shale development are the Early Syn Rift (Lacustrine Brownshale) and Mid Syn Rift (Fluvio Deltaic Brownshale) zones.

The facies model for the depositional environment of the Pematang Group Brownshale formation used the approach of the Changsong (1991) model, where the well BS-03 was analogous to the Deep Lacustrine Deposits (DF) facies model, as shown in Figure 3.

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 Table 4 it can be seen that at each depth interval the percentage of the main mineral content obtained is Quartz, Clay, and Carbonate, so that the brittleness index can be calculated using the Jarvie equation (2007). From the results of XRD (bulk) analysis, an interesting phenomenon was found, namely the presence of sillimanite and kaliophilite minerals in the Brownshale formation which was significantly started at a depth interval of 10,780 ft downward, both of which belong to the brittle mineral category [25].

From Table 4 based on minor minerals group, there is an interesting

phenomenon, namely the presence of sillimanite and kaliophilite minerals which significantly appear starting at a depth interval of 10,780 ft downward, where both minerals have a brittle tenacity [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.

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.:

a) At the depth interval of the sand-shale series, sillimanite is present, 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%.

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.

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:

The dominance of kaolinite and illite minerals as non-reactive minerals (hard - brittle), where both minerals are products of clay diagenesis, the results can be seen in the ternary diagram (Figure 7), namely smectite due to the influence of high temperature and the presence of potassium (K⁺) minerals, which is supported by the sand-shale series environment changes to illite (kaolinite), silica, and H₂O with the following chemical reactions:

Smectite + K⁺ → Illite + Silica + H₂O

This reaction will produce water (H₂O), causing an increase in pore pressure [15] [28].