Thermal History of Miocene – Pliocene strata in the Well LKB, Kutei Basin, Indonesia

Temperature is the most important parameter in hydrocarbon generation. Well LKB was drilled to test a section in the Sanga Sanga area of the Neogene Kutei Basin in East Kalimantan. The Sanga Sanga Block contains four large to giant hydrocarbon fields in mid- to upper Miocene deltaic sandstones of the Mahakam Delta, Eastern Kalimantan (Indonesia). Well LKB was drilled to a total depth of 2286 m within a Miocene deltaic sequence. Following the measurement of vitrinite reflectance, the samples are scanned in fluorescence and white light modes to obtain sample descriptions. The mean maximum vitrinite reflectance data provide a basis for inferring some aspects of the thermal history of the sedimentary sequence in well LKB. The well intersected with a small portion of the oil mature level. The conventionally defined principal zone of oil generation (oil window) probably lies in the section from about 1066 to 3962 m. The measured maximum formation temperature at 2286 m is 93,9 ^°C. Assuming a surface temperature of 26 ^°C, the bottom hole temperature (BHT) corresponds to a geothermal gradient of 34,55 ^°C/km. Temperature measurements at intermediate levels give a range of geothermal gradients. The highest temperature gradient results from a single mesurement at depth of about 225 m which would indicate a geothermal gradient of 158,1 ^°C/km. This value is more than three times higher than the gradients obtained from any other measurements downsection. The causes of this extremely high thermal gradient is unknown. Although the presence of faults, overpressured zones, and flux of hot formation water that expelled from deeper parts of the section are possible mechanism, such an extreme geothermal gradient may also be a product of measurement errors that may be related to older reworking and hence it would be more matured. The average geothermal gradient calculated from the other five measurements is more normal by 38,5 ^°C/km.  Modelling the measured levels of maturation using the logging run temperatures gives maturity levels different from those observed. A compromise model that approximately simulates the observed maturity levels can be obtained by assuming a cover loss of about 914 m of sediments since Pliocene, and geothermal gradients respectively of 38 ^°C/km and 30 ^°C/km for pre- and post- Pliocene times, respectively.