5. Capitalized Exploratory Well Costs

The following table discloses the net changes in capitalized exploratory well costs pending determination of proved reserves during the three months ended March 31, 2018 (in millions):

Balance at January 1, 2018	$	304
Additions to capitalized exploratory well costs pending the determination of proved reserves		42
Balance at March 31, 2018	$	346

Capitalized exploratory well costs capitalized for greater than one year following completion of drilling were $211 million at March 31, 2018 and primarily related to:

Gulf of Mexico: Approximately 55% of the capitalized well costs in excess of one year relates to the appraisal of the northern portion of the Shenzi Field (Hess 28% participating interest) in the Gulf of Mexico, where hydrocarbons were encountered in the fourth quarter of 2015. The operator is conducting appraisal activities on adjacent acreage and is evaluating plans for development of the northern portion of the Shenzi Field.

Guyana: Approximately 20% of the capitalized well costs in excess of one year primarily relates to the Payara-1 well on the Stabroek Block, offshore Guyana (Hess 30% participating interest), where hydrocarbons were encountered. The operator plans to integrate this discovery into the third phase of development, with first production from this phase planned for late 2023 or early 2024.

JDA: Approximately 15% of the capitalized well costs in excess of one year relates to the JDA in the Gulf of Thailand (Hess 50%) where hydrocarbons were encountered in three successful exploration wells drilled in the western part of Block A-18. The operator is currently conducting subsurface evaluations and pre-development planning to facilitate commercial negotiations with the regulator for an extension of the existing gas sales contract to include development of the western part of the Block.