Jan 16 (Reuters) - Reliance Industries, led by billionaire Mukesh Ambani, missed quarterly profit estimates, hurt by weakness in its oil and gas business, higher costs and slowing earnings growth at its retail segment.

The company faces headwinds across its portfolio, including maturing gas fields, tougher competition in its retail business, and evolving geopolitical risks that could hamper its energy segment.

Festive discounting, investment in hyper local delivery and a one-off impact from India's new labour code trimmed core margins at its retail unit to 8% from 8.6% a year earlier, the company said.

Core earnings for the segment grew 1.3% to 69.15 billion rupees, compared with 9.5% growth in the year-ago quarter.

Meanwhile, the oil and gas segment took a hit from a drop in output and softer price realisations from Reliance's ageing KG-D6 fields in Andhra Pradesh.

The flagship deep-water block developed with BP once met a sizeable share of India's gas demand, but production has declined as reservoirs matured.

Revenue from the oil and gas segment fell 8.4%, while core earnings dropped 12.7% to 48.57 billion rupees, partly due to higher maintenance-related costs.

Total expenses for the company rose 11.5% to $27 billion, outpacing a 10.5% increase in revenue to $29.65 billion.

Consolidated profit came in at 186.45 billion rupees ($2.06 billion) for the quarter, missing analysts' average estimate of 196.44 billion rupees, according to data compiled by LSEG.

The oil-to-chemicals (O2C) segment of Reliance Industries, which operates the world's largest refining complex, reported an 8.4% rise in revenue and a 14.6% jump in core earnings.

The O2C business is typically exposed to swings in crude prices, refining margins and global demand, with recent geopolitical disruptions and sanctions reshaping trade flows and sourcing strategies.

We have faced instances where sanctions were imposed suddenly and we had to cut back, Srinivas T, chief operating officer, refinery and marketing, said in a post-earnings call.

But the company took actions, including approaching the national oil companies much ahead of the curve to source the oil without affecting the spot market in any major way, he said.

The company, which has been considering an initial public offering this year for its digital unit Reliance Jio Platforms, said core earnings from the unit rose 16.4% for the quarter.

($1 = 90.7260 Indian rupees)

(Reporting by Chandini Monnappa, Abinaya V, Nandan Mandayam and Sethuraman NR in Bengaluru; Editing by Mrigank Dhaniwala and Shinjini Ganguli)

By Chandini Monnappa