Distribution of Synrift Lacustrine Source Rocks and Reservoirs in Continental Rift Basins

Abstract

Outcrop observations in several Triassic rift basins in northeastern Thailand combined with well, core, and depositional environment maps from twenty six Tertiary rift basins in Southeast Asia indicate that the distribution of synrift lacustrine source rocks and reservoir sequences are remarkably similar in continental lacustrine rift basins. Based on those observations, we can define a succession of eight synrift sedimentary facies that comprise the continental synrift petroleum system. This paper will review both the lateral and vertical distribution of these facies in a continental rift basin sequence to provide interpreters with a predictive model for exploring synrift sequences.

Basal conglomerates

The basal synrift conglomeratic alluvial deposit is a conglomeratic bed observed near the contact with the underlying pre-rift section.

Lacustrine Shale

In the structurally lowest portion of the rift basin sequence we observe a series of source-prone fissile planar bedded black algal-rich carbonaceous shales.

Fan delta Facies

The fan delta facies is found on the steep flank of asymmetric half grabens. This facies was deposited adjacent to the horst block and is generally comprised of coarse grained alluvial sandstone.

Lacustrine Delta Facies

Moving toward the rift basin margin we observe a laterally extensive coarsening upward sequence of planer bedded fine to medium grained sandstone beds interbedded with prodelta shales.

Lacustrine Swamp Facies

A thick section of source prone carbonaceous shales, peats and coals are observed on strike with the proximal deltaic facies. These beds were deposited in a swamp environment adjacent to the deltas along the lake margins.

Shoreface Facies

An extensive shoreface facies develops along the lake margin due to reworking of the deltaic sandstones as the lake level fluctuates. Individual beds are laterally extensive with little change in thickness.

Fluvial Facies

Fluvial sandstones are observed along the rift basin margin. These fluvial deposits can consist of meandering river deposits but more commonly consist of braided river deposits.

Transitional Red Beds

A sequence transitional from synrift to post-rift is observed in many rift basins. This sequence is often a major reservoir sequence. It is typically comprised of braid plain red bed deposits. These are deposited as the rate of basin subsidence wanes while sediment input rates remain relatively high.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018