## 1 Introduction  
Reservoir characterisation is a process that combines all the available field data from various data sources to characterise different reservoir properties in spatial variability quantitatively [1]. These properties play a significant role in petroleum field operations and reservoir management [2, 3]. The main data sources include seismic, well logs, and core data. Integrating these data sources improves the reservoir characterisation process and reduces uncertainties [4, 5]. Moreover, integrating various techniques showed effectiveness in reservoir characterisation. Such approaches tend to reduce uncertainties and enhance reservoir properties' prediction [6, 7]. The exploration of high-pressure and high-temperature (HPHT) fields is a current leading-edge in oil and gas commerce. The expanding need for hydrocarbons around the globe has driven the search for new resources in the exploration and production industry. The new resources are mostly located in deeper formations as earlier explorations regularly focused on shallower formations. Drilling into deeper formations may result in encountering higher pressure and temperature zones. HPHT fields have widely occurred in the Gulf of Mexico, the North Sea, Southeast Asia, Africa, and the Middle East. Researchhas projected that from over 100,000 wells drilled worldwide in 2012, almost 1.5% of the wells can be classified as HPHT [8]. These wells are frequently discovered in regions where the exploration of new horizons is in progress. Although the number is relatively small, the wells usually indicate important potential resources. According to the American Petroleum Institute (API), a well is defined as HPHT when it encountered pressure more than 15,000 psi (103 MPa) and the temperature exceeds 350\({}^{\circ}\) F (177 \({}^{\circ}\)C) [8]. Figure 1 demonstrates an in-depth classification of HPHT specifications by Schlumberger. The classification is based on a structure that considers the thermal stability of design components, appropriate electronics, and pressure ratings of hardware. Based on this classification, the HPHT operations are segmented into three main tiers. Tier I refers to the HPHT zone for the wells with reservoir pressure within 10,000 psi to 20,000 psi and reservoir temperature between 300\({}^{\circ}\) F to 400\({}^{\circ}\) F. Tier II is defined as ultra HPHT, which involves any reservoir with pressure exceeding 20,000 psi and less than 35,000 psi and temperature amidst 400\({}^{\circ}\) F to 500\({}^{\circ}\) F. Tier III involves extreme HPHT wells, with reservoir pressure ranges from 35,000 psi to 40,000 psi and temperature within 500\({}^{\circ}\)F to 600\({}^{\circ}\) F. The HPHT-hc is categorised according to situations that rarely occurred in oil and gas wells, even though geothermal wells may surpass 500\({}^{\circ}\) F and several deep-water wells have downhole pressure that exceeded 35,000 psi [8]. HPHT conditions influence each phase of production and development, which involves the drilling risers, sensors, safety control valves, and blowout preventer (BOP) control systems. Due to high-pressure conditions during completions, testing, and production operations, a possibility of hazard to personnel working with equipment may occur. To administer the exposure to hazard and permit safe wellsite operations, the engineers work with appliances solely constructed to operate above the predicted maximum pressure. Before using equipment, they are assessed above the highest predicted pressure to ensure operations can be performed safely [9]. This research focuses on identifying the hydrocarbon potential of the HPHT zone of the Sepat field, located in Malay Basin using seismic and well log data by applying rock physics study, simultaneous seismic inversion, and ANN. The rock physics study mainly involves cross-plotting of several rock properties. Cross-plots are a visual description of the connection among two or more parameters that enable simultaneous and meaningful evaluation of two attributes with ease.