Geothermal Gradients in the Upper Amazon Basin derived from BHT data

Abstract

The Upper Amazon Basin (UAB), present foredeep of the sub Andean retro-foreland basin. It comprises Putumayo area (southeastern part of Colombia), Oriente area (eastern Ecuador) and Marañón area (northeastern part of Peru). Bottom Hole Temperature (BHT) from a regional well log database (1076 wells and 2957 logs) were analyzed and data discriminated according to drilling operations (i.e., log acquisition operations, cementing, formation test, tools misreading values), topography and shallow subsurface weathering conditions (i.e., temperature data from wells with depths below 305 m. were avoided). A new normalized database has been setup (1021 wells and 1399 logs). Analysis of this data set has allowed better understanding of the regional distribution of geothermal gradient variations within the study area. The results indicate mean uncorrected geothermal gradient (UCGG) for the UAB of 20.4 °C/km. The UCGG is a first approach based on well data with sufficient information and is useful for comparison purposes with other basin where corrected data is limited. In addition, a new computer-generated contour Geothermal Gradient Map (GGM) has been created, using 56 locations (for which number of BHT at different depths ≥ 3, and temperatures in the range 23.4 to 44.4 °C). As for the locations 2 are in Colombia, 28 in Ecuador and 26 in Peru. This map is useful in analysis data of UCGG due to its wide distribution along the basin. Finally, correction based on Horner´s method was applied to these datasets (where number of BHT ≥3 at the same depth; time since circulation - TSC incremental), obtaining a Corrected Geothermal Gradient (HCGG) of 22.9 °C/km (46 wells and 153 logs). We recommend the use of this gradient for comparative reference purposes.